You should clean and blow out the dirt as well as you can first, then spray some WD 40 on the ring.
Then use a large screwdriver as a punch on the tangs sticking up. Drive the ring counter clockwise by tapping the tangs a bit, one at a time.
I'm sure there is a kent moore wrench to remove those rings, but the punching works fine.
The ring rotates like a gas cap. Turn it counter-clockwise about 1/4 of a turn and the unit will come right out. It's a good idea to to get a new gasket (rubber O-ring about the size of a silver dollar...$2.50) and change the mesh filter at the end of the tube ($10). Or maybe you're changing out the whole sending unit assembly anyway.
Here's a good question...does anyone have the best method for cleaning out the inside of a gas tank?
Depending on the vintage of the tank, you might want to also have another metal ring handy in addition to the o-ring. When I replaced my tank, I wound up getting another because it would not break free. There can be alot of friction betwenn the o-ring and metal ring.As far as cleaning, if it is grit, etc, then a rinse of gas will do the trick. Do it a few tomes. If rust or scale, then use a tank cleaner like Eastwood sells or POR15.
In regards to the question about cleaning out an old tank, I like to put a section of old chain inside the tank and then rock that tank side to side and upside down and back and forth.. Hearing protection optional as this makes a heck of a noise. If you have rust inside the tank you might consider using Muratic acid, same stuff you use in pools, sprayed inside the tank and allowed to sit for at least ten minutes. This will disolve most rust and scale. Be sure to rinse the tank with a mixture of Arm & Hammer Baking soda and tons of fresh water. The baking soda will neutralize any remaining acid. Rinse repeatedly and dry with whatever method necessary. Be sure to use one of the tank sealing products as sold by Eastwood or others. These sealers will prevent future rust and scale.