Here's the steps (my non-computer-savvy girlfriend had to figure this one out for me, but unlike all the far more complicated processes one can find online, this one works)

1. Open Control Panel
2. Click "Uninstall a Program"
3. Scroll down to Python and click uninstall for each version you don't want anymore.

This works on Windows 7 out of the box, no additional programs or scripts required.

Open Settings → Apps → Apps & Features.

.

Uninstall Python from there.

Solution:

UNHIDE ALL FOLDERS BEFORE FOLLOWING DOWN...

1. Go to C:\Users\%USERNAME%\AppData\Local\Programs
2. Delete Python Folder
3. Go to Control Panel >> Uninstall a Program
4. Right Click on Python and then Change/Modify
5. Click on Repair Python. Note: This may or may not fail, but be patient
6. Now Again go to step 3
7. Now, after step 3, uninstall Python

Now you should be able to install a new fresh version.

in case you still need it.

first of all, it's never advisable to uninstall python. check for instance this link.
having said this, Ubuntu 16.04 does not depend (wasn't build) on Python 3.9 (check this out, for instance). thus, and in principle you shouldn't be having problems fully uninstalling it... as it was my case.

i had no troubles following these steps; which for you/me would be something like:

sudo apt-get clean
sudo apt-get autoremove --purge
sudo apt-get remove python3.9
sudo apt-get autoremove