You basically unplug everything and unscrew it. Just don't unscrew the riser screws underneath it from the case. You'll need those there for the next motherboard.
The port shield on the very back might be a little tricky but it should pop out (well, it pops inward).
You will need thermal paste to apply to the heatsink / CPU on the new motherboard, unless it comes pre-applied and you're not overclocking. If you're re-using the CPU or heatsink clean all the old thermal paste off with alcohol before applying any new paste. Be aware that thermal paste will stain clothes.
Also make sure the new motherboard you're getting matches the size of the previous one. There's ATX, mini-ATX, micro-ATX, etc. If you're just getting another of the same motherboard then this shouldn't be a problem.