For the sending unit, remove the fuel tank from behind the seat. Carefully remove the locking ring and old sending unit. Reverse the steps to replace it with a new sending unit.
For the gauge on the dash, you may be better off purchasing an aftermarket unit and mounting it somewhere easily visible. It should come in a kit with instructions, but basically you will need to run power, insert a resistor in-line and send power to the tank-mounted sending unit. If you want to replace the OEM gauge, you'll need to remove the 4 screws from the dash panel and remove the panel, disconnecting the wiring harness and speedo cable. There should be 2 screws securing the gauge to the dash. Remove those and reverse the process to mount the gauge and reinstall the dash. You will most likely need to calibrate the gauge using a small inline resistor to make sure it reads properly.