Reactivating a cancelled subscription is now very easy for your customers to do on their own. They will first need to log into their account on your store, where they will see their cancelled subscription, with a new option to 'Reactivate':
Clicking this button will take them to a page where they can choose the modify the terms or variants on their subscription. Their current options will be the defaults:
After selecting to Reactivate with the shown options, they will get a Confirmation page, where they will see the billing price, and will have to confirm.
Finally, they will be taken back to their account page, where they will see that their subscription has successfully been reactivated.