Private Sub Form_Timer()
    ' the Timer Interval has been set at 1000, that is, once per second.
    
    Static bFlag As Boolean
    If Me.Dirty Then
        If Not bFlag Then
            Me!cmdUndo.Enabled = True
            bFlag = True
        End If
    Else
        If bFlag Then
            Me!txtFirstName.SetFocus
            Me!cmdUndo.Enabled = False
            bFlag = False
        End If
    End If
End Sub

Sub cmdUndo_Click()
    ' same action as clicking Undo from the Edit menu
    DoCmd.RunCommand acCmdUndo
End Sub

Private Sub Form_Timer()
    ' the Timer Interval has been set at 1000, that is, once per second.
    
    Static bFlag As Boolean
    If Me.Dirty Then
        If Not bFlag Then
            Me!cmdUndo.Enabled = True
            bFlag = True
        End If
    Else
        If bFlag Then
            Me!txtFirstName.SetFocus
            Me!cmdUndo.Enabled = False
            bFlag = False
        End If
    End If
End Sub

Set TimerInterval to 1000

Add a command button called cmdUndo, which will be enabled the moment the form is edited.

Detect if form is being edited - Option 2 (timer)

form

Visual Basic