If you have early access client, it is exactly the same as the final release. You'll get some lightweight patches if you haven't run it in the last few days. So you don't need to uninstall it.

As for PC, you should be able to log in to Mog Stations and "Add a platform" (the meteor with a plus sign) and apply the registration code (case sensitive). I am not about the Windows version, but definitely check the Mog Stations.

Also failing that, read the instruction manual, it should say there - other than the bit of creating new Square Enix Account (or service account).