Go to "Team Settings":
Click on "Payment":
Click on "Cancel Subscription":
This will open a window. Please carefully read through it and follow the instructions it provides:
To cancel, type in your Team name in caps, click on the check box, and click "Yes, Remove It!".
When you cancel your Subscription, your time remaining in Premium will be outlined on the "Payment" page:
To see what happens when you cancel your Premium Team subscription, click here.