You will need to create an account in iStall first. See here for more details on creating an account.
Once you have created an account and verified your information, sign in to your iStall account settings. This can be done by clicking the gear icon in the top right hand corner and hitting Sign In. If you are having trouble signing in, see here for more details on resetting your password.
Skip this step if you clicked directly on the (account) link above. Once you are logged in, under the settings icon, click on My Account.
Under My Account, click on Manage Credit Cards, or simply click the button below.
You will then see a listing of your current credit cards. You can either edit your current credit card details. You can also delete old credit cards and add new ones.
Now that you have updated your credit card, make sure to update any monthly billed parking registrations.
For a tutorial on how to update your recurring billing, click here. To update your recurring billing now in iStall, click below.