Can't update from 5.0.1 to anything

Hi guys, 2 days ago I got this phone off-contract, and I can't update it. At start I filled it up with a bunch of software to check it out, and today I did a soft reset, as I read that it can't help if you can't get an android update and I wanted to get rid of all the stuff I installed.

Thing is, when I go to "check for updates", it says my device isn't registered. I tried 2 soft resets so far and no luck.

Any thoughts on "how to register"?

Thanks!