This is all done from the Member Center.

1. Go to the Member Center and sign in.
2. Locate the My Membership block. It's to the right on larger screens and towards the bottom for smaller screens.
3. Click on the purple button in that block. It may say "Get Premium", "Renew", or "Manage Subscription". (Examples below)
   

   

4. Follow the instructions on the pages that follow.