If you have collected the membership payment via Stripe you will be able to click the refund from within TidyHQ.
View the transactions at Finances > Transactions.
From the Actions menu (on the right hand side), select Refund.
The information about the transaction will appear in a popup box. Select the Refund button to refund the amount required.
Note: All monies (including fees) will be refunded to the Contact (except in the case where a partial refund is made).
If your Contact has paid via other means you will need to refund them manually.
To Cancel one of their Memberships head to Memberships > Reports > Click on the Number next to the relevant Membership and click the Cancel button on the right side of the subscription details.