I was having the same issue as the older python executable was in my system space (which I have no access to) and the newer version is in the user space.

The work around I thought of was to create a .bat file which will open a CMD window in the user space python version.

new_python.bat:

@set "PATH=C:\Users\USER\AppData\Local\Programs\Python\Python39\;C:\Users\USER\AppData\Local\Programs\Python\Python39\Scripts\;%PATH%"
@cmd /k python --version

Does a decent job for me. Just change the first two lines in the PATH variable to your Python directory and the Python Scripts directory. The CMD which will open using this bat will be ready to run the newer version of python executable.

Easy solution. My last installation was python 3.6 (folder name Python36-32). I just changed folder name to something like Python36-32- and now python 2.7 running.

It depends on your operating system. If you have python 2.6 installed, you need to change your environment path to point to the 2.6 executable rather than the 2.5 executable. Do a Google search for changing your PATH variable on your operating system.

After doing a restart of my machine after deleting python34 from my system path variables as @Talha Junaid mentioned in the comments, my python now defaults to python3.6 when accessed through command prompt, windows must load all the path variables on startup and hold them in memory until a restart occurs.

Steps for anyone else starting the process.

Windows 10

1. Type "System" into the search bar to go to system settings in the control panel (Control Panel\All Control Panel Items\System)

2. Click "Advanced System Settings" -> "Environment Variables"

For User Variables:

3. Highlight the row for "Path" by clicking on it and then click the "Edit" button
4. Highlight by clicking the version of python you want to remove from your environment variables and then click the "Delete" button.

Repeat steps 3 and 4 for System Environment Variables.

5. Close all the windows by clicking the "OK" buttons

6. Restart your machine

To set the default version of Python, you need to have installed Python Launcher (cf below for some info). If you didn't, you can probably install it by using the installation .exe of one of your Python installations, and selecting modifying/repair.

You also need to know where your Python Launcher is:

• if you installed it for all users, it is in C:\WINDOWS\,
• if not, it is in %USERPROFILE%\AppData\Local\Programs\Python\Launcher\.

In this example, the latest installed version of Python is Python 3.11 but the version that should be the default one is Python 3.6.

To set a default Python version:

• go to your Python Launcher Folder and create 2 files: py.ini and pyw.ini

• make sure that both of these files are UFT-8 (they shouldn't be UFT-8 with BOM)

• add to each of these files:

   [defaults]   
   python=3.6
  

• now, right click on any .py, select open with, select check another app, check Always use this app..., scroll all the way down and click on more app... and navigate to your Python Launcher folder and select the py.exe.

• do the previous step again but this time, right-click on a .pyw file and select the pyw.exe.

How to use it

● in a CMD:

• Do not use anymore python in CMD, always use py. If you type just py, then it will open a Python 3.6 console. If you want a Python 3.11 console, type py -3.11.

• As indicated in the comment by @karl-knechtel: If you have an active venv, using python instead of py` will prioritize the venv's Python, which is generally what you want.

• To use pip to install :

  py -3.11 -m pip install # will install package for 3.11 py -m pip install # will install package for 3.6

● For python files:

• If the file doesn't contain a shebang, then it will be started with the default Python version which is 3.6.
• To use Python 3.11 for this file, add the shebang #!python3.11 at the very top of your script

To verify that everything is working:

• open a CMD, write py -0p and look for the *. It should be next to the default version you have set in the .ini files.

  Installed Pythons found by py Launcher for Windows
  -3.11-64 C:\Users<user>\AppData\Local\Programs\Python\Python39\python.exe
  -3.6-64 C:\Users<user>\AppData\Local\Programs\Python\Python36\python.exe *

• With this setup, you can use the specific version of Python that you need in your scripts:

  • 3.6 (as it will call the default version)

    import sys   
    print(sys.version_info)  
    input("close")  
    

  • 3.6 (as it will call the default)

    #!python   
    import sys   
    print(sys.version_info)   
    input("close")   
    

  • 3.11

    #!python3.11   
    import sys  
    print(sys.version_info)   
    input("close")   
    

  • 3.11 (as it is the latest python 3 installed)

    #!python3    
    import sys   
    print(sys.version_info)   
    input("close")   
    

Some info about the Python launcher from docs.python.org

The Python launcher for Windows is a utility which aids in locating and executing of different Python versions. It allows scripts (or the command-line) to indicate a preference for a specific Python version, and will locate and execute that version. Unlike the PATH variable, the launcher will correctly select the most appropriate version of Python. It will prefer per-user installations over system-wide ones, and orders by language version rather than using the most recently installed version.

Two .ini files will be searched by the launcher - py.ini in the current user’s “application data” directory [...] and py.ini in the same directory as the launcher. The same .ini files are used for both the ‘console’ version of the launcher (i.e. py.exe) and for the ‘windows’ version (i.e. pyw.exe).

Customization specified in the “application directory” will have precedence over the one next to the executable, so a user, who may not have write access to the .ini file next to the launcher, can override commands in that global .ini file.

⚠ Your python version can be installed either for all user (python.exe will be in program files) or for the profile (python.exe will be in appdata). If it doesn't work, try uninstalling all python version, and the launcher and reinstall everything as current user to start clean. To do that, download the .exe of your current Python installation, run it and click on uninstall (dont delete the remaining folder in app data if you want to keep your python packages), then reinstall it. If you install the launcher for all users but python for current user, your python.exe will be in appdata (a folder for each version), and your launcher will be C:\Windows\py.exe

If you want to install it for all "all users" and the check box is grayed out: you need first to uninstall the python launcher. If it's still grayed uninstall the python version(s) that remains (everything wil be in program files).

You should have python3 also in your path. You could use python3 to run your script:

python3 <your_script>

But to answer your question, you could use alias and update it as python.

$ python --version    
Python 2.7.6
$ alias python=python3
$ python --version    
Python 3.6.4