ok..tell us what ECU you have...to do this is very simple....

1. remove your seat
2. take your keys out of the side panel, and undo the top and bottom bolts on this side panel (you will need a 10mm socket).
3. Pull out the rear-most corner of your side-panel, so it comes out of the grommit.
4. now, slide your side-panel rearwards, so that the two "ears" come out of the petrol tank cover...and Voila!

Your ECU is the box nearest the petrol tank, just read off the numbers on the sticker. If you get stuck, have a look at the photos on this link....to show you what the ECU looks like, and the numbers needed.
http://www.xt660.com/showthread.php?t=8546

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2008 XTR Mods : DNA Stage 3 Airbox, PC111, Carbon Cans Ovals, Steve Ruth built XTR734cc Big Bore with - High Desert Dyno (HDD)/ Cuervo Racing Gas Flowed / Ported Head fitted with +1mm Valves & uprated Springs, HDD 5050 Camshaft, HDD +3mm Throttle Body, CP 105.5mm 11:1 Piston, Custom Mapping, Piasini ECU, Renthal 613-01, Barkbusters VPS, Pivot Pegz, Touratech Bashplate, Foam Grips, Kev's O2 Sensor Mod, Kev's Extra Coolant Cooler, Wilbers Progressive Fork Springs and Rear Shock