Just make a new stamping in the correct direction within one minute of the previous one, doing so in "Presences" only the last stamping will be maintained as correct. If you cannot act during the first minute, please report the error to email@example.com.
If you are still near the terminal you can follow the "Transactions" procedure described in the document "User Guide" . If you can use a PC, you can check directly in your "MyUnitn" in "Presences" widget "Today's Stamps"
Yes, the procedure is described in the "User Guide" document attached to the following page.