To change your subscription settings, head to the Subscription section of your Settings page.
If you are on the free Apprentice account, this page is the place to upgrade to a Guildmage account. You can select a monthly or yearly plan (the yearly plan gives you two free months!) and your payment method - Stripe for credit/debit cards or PayPal.
Alternatively, you can also upgrade by entering a promo code. Enter your code into the box at the bottom of the page and then click Claim.
If you are a Guildmage, this is the place to update or cancel your subscription or take part in the CardCastle referral program.