Start on the sign-in page. The import asks for your purchase code: on CodeCanyon it is under Downloads › Download › License certificate. If you have no account yet, the import creates one with your Envato e-mail address, and your CodeCanyon licence shows in your account next to any other licences.
Once it is imported, you get 50 % off the first year of a yearly licence, once per licence; the offer is open for six months from the 4.0 release. The upgrade page has the steps that follow.