Yes, you can downgrade at any time, and will keep your premium privileges through the end of your pay period.
If you wish to downgrade your plan, you can do so by following these steps:
- Login to mail.onmail.com
- Tap your user image/avatar in the top-right corner
- Tap Settings
- Tap Account
- Next to the "Personal" plan, tap the Upgrade button.
If you have any issues getting this done, or have any other questions, please let us know!