I have been using Debian for literally decades and I didn't even know "apt-get upgrade $PACKAGE" existed. It is weird, it doesn't appear in the documentation, it doesn't work with the "apt" command, it means it is probably a relic of the past left there for compatibility reasons and you probably shouldn't use it.

My guess is that someone or some LLM hallucinated this command, "apt-get upgrade" is for upgrading your system, not for upgrading a single package, and it takes no extra argument.

For upgrading a single package, just do "apt install $PACKAGE". It is the same command as for installing. The semantics is rather clear to me, upgrading is like installing the new version on top of the old version. It also makes no sense to install a package you already have or to upgrade a package you don't have, but if you want to be sure, for example because you don't know if you already have the package installed or not, there are the --no-upgrade and --only-upgrade options.