There have been a few questions lately on problems with
tlmgr on Linux (specifically Debian/Ubuntu) being the wrong version ("you cannot use TL2021
tlmgr to update TL2022" or similar).
Is it worth having one 'definitive' answer (as a Community Wiki), which we can refer people to? I can see from the often referenced questions that there is a (quite old, but somewhat detailed) description of installing TeXLive on Debian. While this is fine for installing, it doesn't seem to cover the problems with attempting to install the latest version when there is a previous version already present. A few questions have had the problem of a previous version being there, unknown to the user, as a dependency of something else due to the package manager, and the previous version is generally the one selected by
I am happy to write up a question and start the answer if people think that it is a worthwhile endeavour.
tlmgrcan't be used to update at least in user mode seems to be the reality of at least the Debian version, but I don't think that's universally true for any packaged version of TeX Live (though you would still be stuck to that TL version if it wasn't kept up to date).