Click the person icon in the top right corner of itopia CAS and select Subscription:

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.

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