You will have to remove the intake, and replace the gaskets (they are reuse able but you should have new ones on hand in case they are damaged.
The spark plugs installed in the engine are designed to last for 100k miles, unless you have a misfire, then there really no reason to replace them.
How to change the spark plugs
To change the spark plugs on a 2005 Cadillac CTS 3.6, first, ensure the engine is cool and disconnect the battery. Remove the engine cover to access the spark plugs, then carefully disconnect the ignition coil connectors and remove the coils. Use a spark plug socket to unscrew the old spark plugs, then install the new ones, tightening them to the manufacturer's specifications. Reassemble by reinstalling the coils and connectors, followed by the engine cover, and reconnect the battery.
It is moderately easy to change the spark plugs on a Liberty, although it is rather tricky because of the gap between the spark plugs.
Sell it !!!
Spark plugs and wires
Changing spark plugs is relatively easy. Disconnect the batter and spark plug cables, and remove the old spark plugs. Next replace the spark plugs with new ones, reconnect the spark plug cables and battery.
To remove the spark plugs on a 2005 Cadillac STS, first, ensure the engine is cool and disconnect the negative battery cable. Remove the engine cover if necessary, then carefully disconnect the ignition coil connectors and unbolt the coils to access the spark plugs. Use a spark plug socket to carefully unscrew and remove the old spark plugs, taking care not to damage the threads in the cylinder head. Finally, replace the plugs with new ones, reassemble the ignition coils, and reconnect the battery.
Have a look at ; www.carbasics.co.uk
This is for the '05 Frontier, but should be what you're looking for. http://www.clubfrontier.org/forums/f23/how-change-spark-plugs-2005-gen-2-frontier-40507/
Disconnect the battery, remove the spark plug cables and boot, and remove the old spark plugs. Put in the new spark plugs, attach the cables and boots, and connect the battery.
usually every 100000
To replace the spark plugs on a 2005 Cadillac STS V6, first, ensure the engine is cool and disconnect the negative battery cable. Remove the engine cover if necessary, then carefully detach the ignition coils by unfastening their bolts and disconnecting the electrical connectors. Unscrew the old spark plugs using a spark plug socket, and install the new plugs by hand to avoid cross-threading, then torque them to the manufacturer’s specifications. Finally, reattach the ignition coils and engine cover, and reconnect the battery.