At this point you currently aren't able to upgrade your Membership. When you complete your initial 12 month term, you are able to cancel your Monthly Membership and take out a new membership on another Monthly Membership package.
Please note: As of 24th September 2020, we have suspended the purchase of Merlin Memberships. We will be launching a pay monthly Merlin Membership option for Gold and Platinum Passes in 2021.