ML430 Gas Meter Read Problem

[mefp43]

I have 1999 ML430. When I get a full tank of premium gas and I am on a smooth horizontal level, the meter doesn't go all the way to the Full mark. The meter points a little below it. Now, if I park the ML pointing upwards on a slope then it will read correctly. But then, if I park her downwards,the meter makes it appear like my engine used up a lot of gas.
Someone told me I need to change the fuel sensor dipped in the gas tank. My vehicle is out of warranty. I can do it if somebody instructs me on how to do it. Thanks. Johnny