From the Billing section in the Account Settings you can:
Upgrade/downgrade plan
Under Current Plan, click Update plan button
Click on Continue button of the desired plan
Cancel subscription
Under the Current Plan section, click on "Cancel Plan"
Update billing details or change payment method
Change Payment Method or Billing and Shipping Information
You can always come back and add or delete more payment methods, by following the same steps.
Download past invoices
Click on the invoice you want to download from Invoice History section