You can do this on your own if you have any knowledge of cars. If not, a mechanic is the best way to go.
How to change the spark plugs
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.
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
NGK platinum plugs
On a V6, yes.
I reach over and around the top of the engine to access the spark plugs.I reach over and around the top of the engine to access the spark plugs.