Change by what? If the mouse is over the button or when its clicked on?
If its just all about changing the bmap of the button while the mouse is over it here is your solution:
Code:
bmap sitbutton = "sitbutton.bmp";
bmap sit = "sit.bmp";
panel panel1
{
pos_x = 70;
pos_y = 600;
layer = 1;
button = 0, 0, sit, sit, sitbutton, null, null, null;
flags = visible, transparent;
}