For the 2003 Cadillac DEVILLE Fuel Level Sensor Replacement procedure, click here. You will also need the Fuel Sender Assembly Replacement procedure, click here.
Have you removed the locknut yet? If not they make a tool for that, but if you have enough elbow room you could use a hammer and brass punch, brass so it won't cause a spark, and knock loose that lockring.
Here this shows how to remove the lockring.
You can purchase the tool like I said, or if you have enough room, use a brass punch and hammer and tap on either of the tabs on the ring I am pointing to.