To manage your Glass Pass subscription first go to your account page:

From here you will see your order history as well as your Glasspass subscription Expiration/Renewal date

Click "Manage Subscriptions" on the bottom right to bring up the control menu:

You can change, update, or view your subscription details with the 4 blue links below or click on the "GameGlass Monthly Subscription" bubble near the top which will allow you to cancel upcoming subscription billing cycles if desired.

If you choose to cancel your subscription keep in mind this only prevents future re-billings, it does not affect the period you've already paid for nor does it trigger a refund. See here for information on our return policy.