Make sure you are logged into BigMarker. Then from your personal home page, choose the Channel that you'd like to upgrade, downgrade or cancel. You need Admin privileges to access a channel's billing plan. There are a few ways to get to the billing section of your channels.
From your personal dashboard:
From your profile settings:
From your Channel Settings:
All options will send you to your Billing Section. From there you can request to cancel your plan or downgrade it to another plan. You must email support to cancel or delete a channel or plan before or on your payment due date.