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 Recurring Payments, or simply click the button below.
You will then see a listing of your current recurring monthly payments. To change to a different credit card, simply click the Edit button next to the relevant parking product.
After clicking Edit you will see a screen highlighting the monthly parking product with a selection box at the bottom containing both your current card as well as any new ones you have added to your account. Click the dropdown box on the grey "Credit Card" area to change the card.
The dialogue that pops up will show you all of the credit cards that you currently have on file with iStall:
The credit card you currently have selected will have a check mark next to it. To change the card being billed, simply click on the credit card you wish to have charged in the next billing cycle, and then click Save.
If you don't see the credit card that you want to use, click here for a tutorial, or simply the button below to add a new credit card to your account.