How do I downgrade/remove foreign packages?

[RobertX]

I am practising on how to upgrade from Linux Mint 19.3 to 20 to get used to the commands needed to upgrade from 20.3 to 21, and I want to do it on Cubic.

At the stage where I have to downgrade/delete foreign packages, I have not found a single guide on how to do so using command line.

Is there a way to commence that step using command line or am I dreaming?

[maskaw]

apt install / remove ?

[RobertX]

This is what I'm referring to when I say "foreign:" https://linuxmint-user-guide.readthedocs...nt-20.html

Here's a quote from that page

PPAs or 3rd party repositories can introduce issues during the upgrade if the versions of the packages they provide are higher than in Linux Mint 20. This can result in unmet dependencies, held packages or resolver issues.

To purge 3rd party packages follow these steps:

1. Launch the Software Sources tool from Menu ‣ Administration ‣ Software Sources.
2. Open the Additional repositories tab and disable all additional repositories.
3. Open the PPA tab and disable all PPAs.
4. Click on the button to refresh your APT cache.
5. Open the Maintenance tab and click on Downgrade Foreign Packages.
6. Select all foreign packages and click Downgrade.
7. Click on Remove Foreign Packages.
8. Select all foreign packages and click Remove.
9. Reboot the computer.
10. Create another Timeshift snapshot.