Not much you can do but bend the rod the sender float is attached to. All gen 2s read full until you use up about a third of a tank. I highly recommend you leave it alone. What really matters is how accurate it is when the tank is almost empty. My light comes on with exactly one gallon left. Then I know I have between 40 and 50 miles left depending on how I ride.
Try this, ride your bike until the light comes on then refill immediately. You should take around 4.5 gallons.
Lastly, get in the habit of resetting your trip odometer every time you fill up. That way you get a good sense of your range. I go max 210 miles between fill ups. I got caught too far away from a gas station once and rolled in on fumes at 250.