The easiest way to upgrade is to buy an additional FOREVER Club membership. For example, if you currently have a FOREVER Club Bronze Membership and want to upgrade so that you are adding between a Silver and Gold Membership to your FOREVER Club account, you would keep your current FOREVER Club Bronze Membership and purchase an additional FOREVER Club Silver Membership.
If you own two different membership levels that were purchased at different times, you will receive your storage rewards at different times. Each FOREVER Club membership rewards storage independently, which you can see when you'll be awarded by going into your Account Settings and navigating into FOREVER Club.
You can always cancel your current Club Membership and purchase a higher or lower level, but please keep in mind that monthly payments made towards a cancelled Club Membership Level will not count towards the Storage award of other Memberships purchased. (Sorry!)