A quick google search reveals this step by step guide.
Step 1. Disconnect ALL wires from xbox, once everything is disconnected hold power button in for 2 seconds to clear any electric charge left in the console
Step 2. Remove Hard Drive and any memory unit/cards you might have plugged into the console. You will remove the hard drive by pressing button on the hard drive and pulling straight up.
Step 3. Remove the faceplate by gripping the top and bottom near the left side of the console(the side that the disc drive is on) and pulling firmly but slowly until it pops off, then repeat this on the other end.
Step 4. Remove top and bottom gray mesh panels by prying them off with any small objects by inserting them into holes on the sides of the xbox console at the left, middle, and right points where you can see the grey tabs and pushing in to release the catches.
Step 5. Separate top and bottom of white case by inserting flat head screwdriver or knife blade into the back of the console at the points shown in the picture. You should hear a pop when each one releases; go down the line starting from left to right. You should hold the side that you're starting on and start to separate the case pieces so that they don't click back together when you pop the tabs
Step 6. Complete seperation of the case by using a fingernail to unhook the tabs on the front of the console starting on one end and working your way to the other side, there should be 4 tabs to release, after that's done the top of the case should just slide right off.
Step 7. Using your Torx T10 star screwdriver remove the 6 screws shown in the following picture, you don't need to remove any other screws even though there's a bunch on there.
Step 8. After all 6 screws are removed and SET ASIDE SO YOU DON'T LOSE THEM, flip the console over so that the remaining half of the white case is on top. Make sure that the metal part that you just unscrewed the screws from is facing down, on the table or ground, etc.
Step 9. Pull the white case straight up, it should seperate easily from the rest of the console, and set it aside.
From here you should see the "guts" of the console, you have the disc drive, cooling fan, motherboard, and a white guard to direct air flow in the console.
http://hubpages.com/hub/Take-Apart-An-XBOX-360