First, excuse me if I am making a wrong assumptions here. I assume you have Thunderbird 78 installed via the Ubuntu package manager and you want to upgrade to Thunderbird 91.
There are, at least, five ways, in principle:
Remove the apt version, and install the Thunderbird 91 snap, by choosing the "candidate" channel instead of "stable". This is a very safe way to upgrade, and recommended.
Find a PPA for Thunderbid 91. I could not find one, and I don't recommend PPAs typically.
Use the downloaded tar-archive from the Thunderbird website. I don't think the "update" is a program you are supposed to run, you should follow the instructions here: https://support.mozilla.org/en-US/kb/installing-thunderbird-linux
Remove Thunderbird 78 with apt
, and use the appimage version of Thunderbird 91.
Yet another alternative is a flatpak, available at https://flathub.org/apps/details/org.mozilla.Thunderbird
I know the instructions for the Mozilla tarball update are outdated, but should still work. It is up to you to place the extracted tar-archive to a place that is convenient for you. I personally would put it in ~/thunderbid and have a symbolic link to the thundebird binary at ~/bin.
You should either a) remove the old Thunderbird 78 via apt/package manager or b) at least make sure you don't accidentally run the old version by creating custom launchers for the new version and setting up your path correctly. I would remove the old Thunderbird completely. Mixing the two versions might lead to some serious weirdness.
apt
package. It does not work that way. The patch you are referring to is to update Thunderbird 78 tarball to 91 tarball version. – Archisman Panigrahi Oct 04 '21 at 08:13