If you've accidentally cancelled a customer's next renewal and the subscription is now set to expire, you can reverse this action using the same dropdown menu you used to cancel it. To do this,

Navigate to the customer's profile > click that dropdown menu to the right of the subscription > click Un-cancel renewal

You'll be prompted to restart the membership on the day it was originally cancelled or today.

Note: You can always save these changes, then return later to edit the day of the renewal by using the Edit option at the top of the dropdown menu pictured above.