If the refund option has been enabled for your account, you can view the transactions you want to refund via transaction overview (“Operations” Menu → “View transactions” → PayID of the transaction). When you invoke the transaction details, you will see a refund button at the bottom of the page. Enter the amount you wish to refund in the text box next to the button (normally the full amount will already be set by default) and then click the refund button.
If you want to refund the full amount or, just once, a partial amount, simply leave the ‘Last’ box checked. If you want to make more than 1 refund on a transaction, however, (e.g. a refund of 50 EUR today and another 50 EUR tomorrow on the same transaction), you need to uncheck the ‘Last’ box.
If your account does not support the refund function, it must be performed directly by your acquirer.