One way to check if the sending unit is the problem is to disconnect it from the electrical connector. If the gauge then reads full, then the sending unit is likely the issue. If the gauge still reads below full, then the problem is likely in the wiring or in the gauge cluster itself.
If the sending unit is the problem, it will need to be replaced. This is a relatively easy repair, but it will require dropping the fuel tank. If you are not comfortable doing this, you will need to take the vehicle to a mechanic.