To add a payment method (card, etc.), just make your first purchase, be it a subscription or tip, for example:
1. Find a creator whose club you'd like to join;
2. On the creator's page, click/tap Join from $...;
3. Click on the payment method at the bottom and choose one of the options;
4. Enter the required information and click/tap Submit Info;
To see what payment method you're using at the moment:
1. Click/tap your profile photo in the upper right, then Account Settings (the tab Account should open by default);
2. Scroll down to the Payment methods section;
3. To remove it, click/tap the icon with three dots, then Delete.
To change your payment method, just delete the current one as descried above and then choose a new option when making a purchase.
⚠️ Removing a payment method does not cancel your memberships, you need to do it manually via the creators' profiles.