Yes. Go to the Transactions page and select the TRANSACTION # for the transaction you would like to modify. Select either Issue Refund or Void. Note that selecting Issue Refund will not automatically refund the transaction, but will first give you the option to select the dollar amount you would like to refund, allowing for partial refunds.
Please note that you can only void a transaction up to 24 hours after it was originally taken. If the Void option is not displayed, the void period has expired.