You can update your payment method by going to 'my account' settings and clicking the 'update payment method' button.
If you are on a desktop/pc:
Click on your account name on the top right of the website (next to the cart and where the current balance is), and click on the 'my account' tab. Alternatively, you can follow this link: https://www.jackpot.com/account-dashboard
If you are on a mobile:
Tap the three horizontal lines on the top left, go to 'my account', 'transactions' and then 'transactions' again. Therein you'll see the 'update payment method' button. Alternatively, you can follow this link: https://www.jackpot.com/account-transactions