1. Insert a coin into the bottom right slot to pry the trim ring off the front cover
2. Remove the trip ring from the front of the Simon XTi
3. Lift up on the two tabs at the top of the panel to open the cover
4. Swing the cover from the back plate
5. Once the panel is open you will have access to where the desk mount tabs snap into the back plate. The first two tabs (circled in red in this picture) should come out easily.
The bottom tab (circled in blue in this picture) you can try to push or pry out, sometimes it is necessary to snap it off with a pair of pliers or cut it off with a utility knife though to get it out.As a last resort you can simply pull on the top of the desk mount and bend it back, it will pop off or break without damaging the panel's back plate.
6. Disconnect the two white power wires from the screw terminals in the back of the panel. Make sure you leave the transformer unplugged when you do this to avoid damaging the power supply. Then you can pull the wire out of the desk mount, back through the back plate, and reattach the two connections to the panel.
7. For the last step, you will need to insert a screw into the wall (or nail) and place the system on the wall. The red arrows will direct you to the mounting holes on the back of the Simon XTi.