If you have the contributions permission on the group, you will see a refund button next to donations or ticket purchases that are eligible for refunds in the donations section of an activist's record.
Click the refund button, then confirm to refund the donation or purchase. The amount will be listed as $0.00 and show up in transaction reports as refunded, and the full amount of the donation or purchase will be refunded to the activist.
Note: It may take a few days for the refund to show up on an activist's credit card statement. If you are using WePay, when you process a refund you give back the money you were given by the activist and WePay gives back its fee, meaning you are not charged a fee on refunds. Rather, it's as if the donation never happened. If you are using Stripe and made your account after September 14, 2017, you will be charged Stripe's fee on refund, though your activist will see a full refund.