I have a server running with Ubuntu 12.10. But now I am unable to update because I need to install update-manager-core which is not possible because my version is no longer supported.
Is there a way to upgrade my server to a newer, supported version or should I let it running as it is (only web- and fileserver).
12.10 → 14.04 is not a supported release upgrade route. The only working route would be 12.10 → 13.04 → 13.10 → 14.04, I think. So you'd either have to perform three release upgrades, of which every one could break your system, or you start anew with 14.04.
Whether you actually need to upgrade is up to you. If you don't want to install any too recent software and the newest features, I don't think you need the upgrade.
However, there's still the security aspect. Ubuntu is a rather safe system (compared to Windows, where pretty much everything is safer), but rather safe doesn't mean absolutely safe. I can't predict how sensible the files on your server are or how you estimate the chance of an attack on your server, but no server is 100% safe. If your server is in an environment with a firewall-protected router, I'd say you're safe enough.
So, again, you have to tell whether you need the upgrade. But if you decide that you need it, the best way is a full reinstallation (Back up your data first!)
My comment would be for this user, Peter Pötzi, to please try to stick to LTS versions in the future in order to avoid this problem. As I recall, LTS versions are supported for five years, meaning that had he installed 12.04 instead of 12.10, he'd not be having this problem.
I manage more Ubuntu installations than I care to count, some as some as old as version 8.10, which hasn't been updated in years and never goes online. I'm currently keeping 12.04 as my primary system and will likely upgrade to 14.04 in a few months, but only after having tried it on alternative media first.
I pretty much agree with everything expressed by the_Seppi above, (except maybe for the ambiguous comment which appears to suggest Windows might be safer than Ubuntu).
