Log in to your account and then click shop, click studio one and look for the upgrade in the list of versions. It might take a few minutes for your registration key to show on your account.
As for Shannon's question, the only real problem I've experience is that a couple of my third party VST's crash in S1 5 pro when they load in a song created in Studio One pro4, like BFD3. So you'll want to check that. But your songs and all that data are stored outside of the installation folder so it should come across just fine. Even though everything should work just fine, I'd make sure all your stuff works before deleting 3 but I would remove 2 pretty quick unless I had a reason otherwise.
