how do i change the rear spark plugs

Either the fuel level sensor, fuel gauge or the wiring/connectors are malfunctioning, hard to say without a diagnosis. The first thing I would do is add a bottle of Chevron Techron Concentrate Plus to the fuel tank and drive it. I have fixed several erratic fuel gauge issues with this stuff. It cleans the fuel level sensor in the tank, give it a shot, it's a very inexpensive repair and it will also help to clean the injectors and valves as well. http://bit.ly/Techron_Concentrate