Hi ​@TanjaK,

you should find the payment in the guest profile under the tab Payments (we work with the German version, maybe my translation is not correct).
There you can either refund the entire amount or a partial amount using the 3 dots on the right.

I assume you are using Mews Terminal?

Good luck

Best regards 
Leif 

Hi ​@TanjaK, I see you received an answer but saw that you meant to undo a refund instead of process a refund.  

If the card is still in Mews (and enabled by Mews Terminal or Guest Portal), you should be able to process the payment again (payments tab in guest profile - methods - select the correct card and click on the 3 dots on the right side to process payment). 

Regards, Sanne 

OH, sorry, undoing the undoing… misunderstanding. You´re right Sanne.