This depends on your university for the most part.
First, go to user.wolfram.com. Can you log-in? If so, do you see version 9.0.1? You can download it here if you have an account. Some univerisities have set things up so you can download you copy of Mathematica this way.
If you can't log into user.wolfram.com, then please contact your university. They should have access to 9.0.1 and be able to get it to you. You don't need a new activation key or anything as long as you install 9.0.1 on the same machine.