A refund puts money back into a customer's account. It allows the record of the purchase to remain and shows another for the money being returned. Voiding a transaction removes the original purchase entirely.
There are several ways to refund a transaction:
Sales/Orders: From the left-hand menu select Sales/Orders and search for the customer, under the column 'items' click on Membership
For one-time memberships you will see a 'refund' button under Order summary
Click on Refund and then fill on the amount and any other details you wish to include:
Members: From the left-hand menu select Members and search for the customer, from the list of results find the customer and either click on their email address or click the settings icon under Action and select View.
You will arrive on the member's profile management page, clicking Memberships will show all current and past memberships for the customer, find the membership in question click on the 3 horizontal dots by the membership price, and select edit.
From the membership details page subscriptions have a payment history with a small orange refund button, clicking refund will open a form to complete the refund, either partially or in full.