There's not much to this, but at the request of Zach I've thrown this real quick guide together.
1. Pull off the cover - I used a nice wide flat headed screw driver and wrapped the head in a cleaning cloth (so as not to scratch anything and gently wedged it into the small gap (found the best one on the drivers side bottom corner) and pried it off.
2. There are 4 bolts which hold the unit in place (red arrows pointing to them in attached pic). I used a socket wrench and a 10mm socket with long handle attachment which is good for reaching the bottom ones which are set in a bit and harder to get at. Undo them all and pull the unit out and rest it on the Gear stick with all the wires attached still.
3. On the side of the unit (no pic sorry) the brackets which hold the unit in place have 4 more bolts (4 on each side). I removed them with an 8mm socket I think?
4. Re-attach the side brackets onto the new head unit so it's ready to go.
5. Take a photo (or use mine) so you can see where each of the wires/connections goes and un-clip them all and lay them out in a sensible order so they're easy to reconnect.
6. remove old unit, and put in the new one, reattaching all the connections. There are 2 connections that aren't directly into the back of the box, they were wrapped in plastic for me. Leave the unit in the same position (resting on top of gear stick).
7. Take the magnetic GPS areal and reach inside the compartment up to the left and find a metal strut that is running in there to stick it to (this is up to you where to put it, but when i put it in originally on the actual box, it didn't have good reception at all - missing by a street all the time).
8. Plonk it all back in and re-bolt it up and put the face plate back on.