To change payment details,
Navigate to Settings -> Billing & Admin -> Billing
Choose Payment Details from the left menu.
PLEASE NOTE: Only the Primary Billing Contact can change payment details,
As a general rule that is the email address that receives the Invoice, in the Admin screen the Primary Billing Contact is shown with the Subscription as Billing managed by Name (Email).
If you are confident that you should have access, try an incognito window as that can bypass other browser extensions that may interfere with the progress of the flow.