fuel gauge sending unit?

so as we all know, from the half way mark to empty on our gas gauge, it goes down rather quicker then it should. but for some reason i figured out, my low gas light doesnt come on lol. thankfuly i wasnt far from home. the bulb in the cluster is fine and i was told somthing about a sending unit or somthing. i was wondering if anyone knows about this or if there is a diy guide on replacing this. my car seems to be getting crappy gas mileage like 20 to 22. idk if thats normal but i always hear people saying they get like 30 with their da’s. anyways, i need help ):