To switch your existing CBS All Access subscription from Commercial Free to Limited Commercials via your iPhone or iPad, please do the following:
1. On your iPhone or iPad homescreen, tap Settings > iTunes & App Store.
2. Tap Apple ID > View Apple ID, and enter your Apple ID password or use Touch ID to sign in.
3. Scroll down. Under "Subscriptions", tap Manage. You will see all of your active subscriptions.
5. Select your CBS All Access subscription, and switch your subscription to Limited Commercials.
If you don't see an CBS All Access subscription but are still being charged, make sure that you're signed in with the correct Apple ID. If you are, then you might have subscribed on another device, or through CBS.com, and not through iTunes. Please return to that device or CBS.com to switch your subscription. Instructions on how to switch your subscription on other devices can be found here