Sometimes ATMs can have problems dispensing the correct amount of cash, but still end up charging your card.
If the incorrect charge is pending in the Revolut app, then it will be reverted automatically after 7 days, and you don't need to do anything.
If it shows as completed, then you should first try to contact the bank responsible for the ATM (if possible) and explain your situation. If the bank is uncooperative or you haven’t received a response, you should fill out online chargeback form so that we can challenge the payment with the ATM operator. Please bear in mind that the chargeback process is a last resort to recover your money and it can take some time for Visa or Mastercard to issue their ruling (find out more about the process here).