Click the person icon in the top right corner of itopia CAS and select Subscription:
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You can edit the existing payment profile, if you have to change out the address or name on the card, but if you're needing to enter a new card number you'll have to create a new payment profile. 

To add a new payment profile hit the green plus sign on the right.
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Fill you the necessary prompts and hit "Create"

Now you'll select Activate on the new profile, and delete the old one. 

Note: Google billing and payment profile is separate from itopia and must be managed from https://console.cloud.google.com/

itopia billing and updating billing email address