On your Bookshelf page below My Subscriptions, click the Monthbook subscription you want to edit.
On the far right side of your screen click the gear for Subscription Settings.
Click on the drop-down arrow next to Subscription title under the subscription settings
Add your new title and click Save
Refresh your screen, and the new title will be visible on your Subscription page as well as your Bookshelf page
If you would like to change the title of your subscription on the Chabtooks app, see here.