Billing & Subscription plans are per Channel, and not per account. To update a billing plan, you first need to go to the specific Channel. You can locate your Channels by logging into your BigMarker account. On your personal "Home Page" you will see "My Channels" on the left had side of the page.
Click on the Channel that you want to update. From the Channel home page, you will see the "Settings" tab. Click that then choose "Billing & Subscription."
If you would like to save a credit card for future upgrades or purchases, go to your personal settings: