With Join It, you can easily refund a membership payment directly from your admin tools.
You can either:
Find the associated Membership and then click on the respective payment at the bottom of the membership record, or
Navigate directly to the payment via the Finances > Payments section (or go here)
Either way, you'll end up on the payment record, which looks similar to the screenshot below.
From there, you'll want to click the 'Refund Payment' link and you'll be presented with two different options:
Complete Refund: Refund 100% of the amount of the payment.
Partial Refund: Refund a part of the amount of the payment.
Once you confirm, the refund will be processed and a refund receipt will appear within Join It (associated with this membership record).