how do I replace the motherboard on a dell xps 600

Look it up on YouTube. If that doesn't help, open the right side panel (right from the back of the case) which is usually held together by two screws. Open it carefully. You will now see the motherboard mounted on the wall of the other panel. You may want to use a grounding band as any static electricity may damage the internals. Now carefully disconnect all the wiring and keep them aside (Label them so that it'll be easier to install them later. Now there will be some screws holding together the motherboard with the panel and after unscrewing them, the motherboard should be separated. Now remove the RAM which will be located on the side of the processor. Do this carefully as there are clips at either side of the socket. After doing this remove any graphics card which has been installed which should be above or below the processor (Depending on how you hold it). Now, remove the CPU cooler by locating and twisting the four plastic screws. These will hang from the cooler. Now, without applying any pressure to the middle, lift the metal stick which should be tucked under a notch and slowly move it upwards. Get the cap out of the way and remove the processor gently and place it in a place which won't damage the underside. Also, don't touch any of the exposed pins from the motherboard or from the processor. Now, get the new motherboard and fix the processor gently but tightly. Without touching or pressing the processor, bring the metal stick down and tuck it under the notch. Now, install the cooler by firmly pressing down on the plastic screws and twisting them, and then connecting the wire to a port labelled "CPU Fan". Also, be careful while placing the processor as there are notches in the motherboard too and if it is placed wrongly then you may damage the processor. Now add the RAM to the slots and press it firmly and close the clips. Add the graphics card (If any) and proceed with the wiring. Don't panic as wiring only looks difficult. Now, fix the motherboard to the back panel (You can also assemble it while you have fixed it to the back panel). Now fix the other side's panel and connect the power supply. Switch on your PC and install the new motherboard drivers.
Also note that you MUST unplug the power supply and ensure that no current is passing through the PC while you are assembling and no cables (Connecting the PC to the monitor or speakers etc. are connected). I will strongly recommend that you watch videos on the assembly and dis-assembly of similar computers.

Hope this helped,
KillerGamer