Easiest way to remove and replace 3.5 engine in 2004 maxima

SOURCE: SPARK PLUG REPLACEMENT FOR 97 MAXIMA

Well, it is usually cheaper then having a shop do it for you. But it will require you to have some tools. In this case probable a 10mm and 5/8 spark plug socket and 6 inch extension and ratchet.

Other than that, you should be good to go.

SOURCE: 2004 nissan maxima rear bulb replacement??

There is a plastic cover on the lens that covers the screws that hold the lamp on to the body. CAREFULLY pry the cover off to access screws, remove the screws and CAREFULLY pry the lens off. Replace bulb(s) reinstall after checking to see if bulbs are in fact working.

SOURCE: 2004 Maxima driving / fog light bulb replacement

if they are near the bottom of the bumper the only way is take the lower shield off and drop the lt and rt splash shields

SOURCE: HOW TO REMOVE THE REAR BRAKE LIGHT LENS TO REPLACE THE BULB

how do i remove my tail light assembly out of my 2005 nissan maxima

SOURCE: 98 Nissan Maxima - Check Engine Light, Knock Sensor

If the car is pinging you will set a knock sensor code.'
This comes from bad gas and running lean., You really need this professionally inspected to rule out the basics. I have seen alot of nissans with the knock sensor code stored but the light is still off.
Fill up the tank with high octane chevron and some techron injector cleaner and clear the light. If it stays off for a couple days then go get it inspected. Even if the code is stored it will still pass as long as the light is still off and the monitors have run.