My 12 year old nephew can change a mainboard out. Its not hard at all.
Precautions:
Like the person above said, either use a anti-static wrist strap or make sure you ground yourself to the chassis before working with it.
Use a quality thermal grease, you don't want a big glob. Just enough to make a thin layer on the CPU dye.
Pull the instruction manual up on the net if you dont have it, so you can make sure you plug all connectors back up that need to be.
If you run into any problems shoot them back.