First, you will need to get the profileId of the player in question.
Your game should be designed in a way where this value is visible toward the player, such as in settings menu or support page. Alternatively, your app may be using a universalId or email to identify the player.
Once you have that chosen identifier, go to the Monitoring | User Monitoring | Virtual Currency page of the brainCloud portal, to search for and select the player.
With the player selected, click the currency value in the "Purchased" column, and enter the newly desired currency amount.
The change on the server will be instant, so all that is to help the happy customer know their refund or reward has been made, and they should refresh their app to view the new balance.