I already have Zandronum installed on linux, do I just do "sudo apt-get upgrade"?
If you've added zandronum as a repository it should work, I personally don't do it, I compile my linux binaries from source.
If apt doesn't work you can get 32 and 64 bit binaries direct from zandronum but keep in mind you may need sqlite and openssl, I'm not sure if those are bundled with it.