You can restart your subscription any time by going on the app, clicking your profile icon on the top right corner, and tapping on the "Restart Subscription" button.
When restarting your subscription, please make sure to check that you are logged in to the correct Manta account that you would like to use for the Manta Unlimited service.
*Down below are screenshots of where you can find the profile icon and where you can find the "Restart Subscription" button :)