Those two tables aren't related to each other. The table you are missing is WHO checked out WHICH set of headphones.
If you aren't tracking that, then all you need to do is have your form update the status of the headphone. The user id has no relationship to the update.
Put a command button cmdSignOut on the form, and assuming the Serial is scanned into a textbox called txtMySerial.
Code:
Private Sub cmdSignOut_Click()
Dim strSQL As string
StrSQL = "UPDATE [Headphones] SET [Status] = 'Signed out' WHERE [Serial] = " & Me.txtMySerial
CurrentDB.Execute strSQL
End Sub
The above is aircode, but it should get you there.
If you DO want to track who checked out which serial, then create that checkout table, use code similar to the above to do an insert to that table with the checkout date/time, then use the above code to update the headphone status. When headphones arrive back at the station, you'd update the status and mark the checkout record with the return date/time.