There are 3 senerios when it comes to the sending unit being faulty.
One, the unit can easily be accessed from the trunk, has an electrical plug, a return line, and a sending line. Pull car battery, unbolt cover, and remove unit, replacing with a new one.
Two: Unit can only be accessed by pulling the tank, not easy what so ever, but once the tank is removed it can be swapped out.
Three: In some cars, the unit is literaly, a part of the tank, and the tank/unit must be replaced.
Most tanks wont leak easily as the newer ones are coated in rubber to help prevent leaks, but if the tank has a major dent in it, it will possibly break the rubber liner.
Check the fuel lines, they run all over and look similar to brake fluid lines, some cars, the fuel lines run along the brake lines. But yea, the float on the sending unit may be damaged, or there is a bad electrical connection somewhere.