After logging into your account, click the gear icon next to the New poll button, then from the screen shown below, click on 'Billing details'.

On the payment billing page, you'll be able to add, update and manage payment methods.

After building a new test and clicking the green 'Preview and pay' button, you'll be able to enter and save your card details or add a new card if this isn't your first test. Please note, this option isn't available if you have prepaid credits on your account to pay for your tests. You'll need to make changes through the first option in settings.