There are a few easy steps, and luckily your customer can do it all on their own.
Note: If you want to make this change for your customer, follow this guide here to login as your customer and complete these steps.
1. Have them log into their account on your site.
2. They will see a list of active subscriptions on this page. Click 'Edit'.
3. Modify the terms and/or variants, or cancel the subscription. Then click 'change'.
When the subscription renews it will now renew with the changed properties!