In the MouseMove event handler, move the point to its new location and redraw the line.

In the MouseUp event handler, set m_GrabbedPoint = -1 to indicate that no point is selected.

Private Sub picCanvas_MouseDown(Button As Integer, Shift As _
    Integer, X As Single, Y As Single)
Dim i As Integer

    ' See if this is a point.
    m_GrabbedPoint = -1
    For i = 0 To 1
        ' See if the point is within the point.
        If Abs(X - m_X(i)) <= GRAB_WID / 2 And _
           Abs(Y - m_Y(i)) < GRAB_WID / 2 _
        Then
            ' This is the point.
            m_GrabbedPoint = i
            Exit For
        End If
    Next i
End Sub

Private Sub picCanvas_MouseMove(Button As Integer, Shift As _
    Integer, X As Single, Y As Single)
    ' Do nothing if no point is grabbed.
    If m_GrabbedPoint < 0 Then Exit Sub

    ' Save the new coordinates.
    m_X(m_GrabbedPoint) = X
    m_Y(m_GrabbedPoint) = Y

    ' Draw the moved point.
    DrawLines
End Sub

Private Sub picCanvas_MouseUp(Button As Integer, Shift As _
    Integer, X As Single, Y As Single)
    m_GrabbedPoint = -1
End Sub