It’s not possible to to change prices of or delete existing subscriptions in Google Play.
The workaround for this is to create a new Subscription with the updated price tier
Once setup, simply untick the old subscription in MagCast (Android Setup menu).
Note from Google about About Subscriptions
“Please note that subscriptions cannot be unpublished, and the price of the subscription cannot be changed.
We are currently working on implementing unpublishing and price changes for the subscriptions.
Additionally, we do not support subscriptions with the price set to $0 (except in the subscription trials).”