I would try cleaning it up manually. I've never done this so make sure you backup beforehand.

• Remove the link from /etc/alternatives
• Remove the relevant file from the admin directory
  • /var/lib/dpkg/alternatives/ on ubuntu (debian may be the same but check the man pages under the FILES section)
  • /var/lib/alternatives/ on CentOS 6&7

replace

[bash:~] $ sudo update-alternatives --install /usr/bin/python python \
/usr/bin/python2.7 2

[bash:~] $ sudo update-alternatives --install /usr/bin/python python \
/usr/bin/python3.5 3

with

[bash:~] $ sudo update-alternatives --install /usr/local/bin/python python \
/usr/bin/python2.7 2

[bash:~] $ sudo update-alternatives --install /usr/local/bin/python python \
/usr/bin/python3.5 3

e.g. installing into /usr/local/bin instead of /usr/bin.

and ensure the /usr/local/bin is before /usr/bin in PATH.

i.e.

[bash:~] $ echo $PATH
/usr/local/bin:/usr/bin:/bin

Ensure this always is the case by adding

export PATH=/usr/local/bin:$PATH

to the end of your ~/.bashrc file. Prefixing the PATH environment variable with custom bin folder such as /usr/local/bin or /opt/<some install>/bin is generally recommended to ensure that customizations are found before the default system ones.

Add python2.6 to alternatives:

sudo alternatives --install /usr/bin/python python /usr/local/bin/python2.6 5

Then set python3.9 as defaut:

sudo alternatives --config  python

Or override the symlink with the desired python version:

sudo ln -fs /usr/bin/python3.9 /usr/bin/python

You are definitely able to do an "n to n" link as you state, just add the python3 group symlink you created as a potential target to python:

alternatives --install /usr/bin/python python /usr/bin/python3 300
alternatives --set python /usr/bin/python3

Each --install command will add or update an alternative, which you can see with alternatives --display python

Do sudo update-alternatives --remove-all gccc.

Compare the python and python3 symlinks in /etc/alternatives/ (e.g. with ls -lF /etc/alternatives/python*.

My guess is that python points to python3.8 while python3 points to python3.10. Or vice-versa.

Is there any particular reason why you want python to run 3.8 instead of 3.10? if not, run update-alternatives again and set it to either auto (preferred) or to 3.10 (you'll run into the same problem in future when 3.11 or 3.12 or whatever is the latest auto version).

Or just make sure that the alternatives symlinks for python and python3 point to the same version of python.

to redirect python to 2.7.3 type in terminal

sudo update-alternatives --config python

and then select the version of python most likely 3

to remove pythn 2.7.5 use apt-get i.e

 sudo apt-get remove python2.7-5

then run

sudo  apt-get autoremove

to remove any dependancies not needed

You need to define the two different python versions

sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.NN 1
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.NN 2

Then you need to choose which version should be defined as default

sudo update-alternatives --set python /usr/bin/python3.NN

Note! You need to check what version you have of python and replace NN with your version.

python2 --version
python3 --version