The need to reverse a credit card charge or refund a donation is very common. A refund will take place in the appropriate payment Gateway or Blackbaud product that supports it.
Please consult your product user guide for refund capabilities within the application. If using a Blackbaud product that doesn't support refund capabilities, issue a refund within your payment Gateway. Once this has been done, adjust the transaction record in your Blackbaud product(s).