Search for your participant by selecting Participants, under Registrations, in the left hand side menu.
Click the search icon in the top right hand corner to view a list of search filters.
Enter the information you have on the participant you want to find and click Apply.
You can then select the participant from the search results and view their information.
In the Financials tab click the Refund button. You'll be given the option to refund the total amount paid or you can choose the amount to refund.