You can and it is fairly simple. To refund a payment, you first look it up in the transactions overview in your account. Next, you click on the 'Refund' button.
In the next screen, you can fill out what amount you want to refund. This can be the total amount or just part of it. Here, you can also add a description. The consumer will see it on their bank statement or in their online banking environment.
The refund will be processed the next working day. With most payment methods, this means that the amount will also be credited to the consumer's bank account the day after the refund. With credit card payments this will typically take a little longer (5 working days on average).