The back spark plugs are at the back of the engine and not very easy to replace. There is a special tool available which you can install in place of the upper engine mounts. With the special tool you can pull the engine forward a bit. With the engine forward remove the spark plug wires, ignition coils, and the evap purge solenoid (make sure you put the vacuum back correctly!). This should allow you to reach over to the back and remove the spark plugs. I would also be a good idea to replace the spark plug wires at the same time. I am sure you won't wish to go back there any time soon to replace plug wires!