On Ubuntu 20.04LTS the app called "Ubuntu software" is in reality snap-store (the terminal command running it is "snap-store", not "gnome-software"). Ubuntu 20.04 does not seem to come with gnome-software installed by default. For me, closing the app, then typing
in a terminal and then re-launching the app ...
sudo update-alternatives --remove-all python3
Remove all alternatives and all of their associated slave links. name
is a name in the alternatives directory (in your case python3)
sudo rm /usr/bin/python3
Then create a new link.
sudo ln -s /usr/bin/python3.6 /usr/bin/python3
The only thing that has worked so far is
But the terminal has to be kept open. Once it is closed, the apps disappear again.
Update: G. Koçmarlı has posted a working solution on Ubuntu software shows only editors picks 20.04.