Full tank-fuel gague registers empty-need fuel light is on

So how do you get a new sending unit in there? and how much do they run? my jeep has the same problem. Also if anyone knows how to fix the rear latch so you can open it that is broken as well. I need to open to fix and to get to my tail lights to replace them. If any one can help please do so soon. So how do you get a new sending unit in there? and how much do they run? my jeep has the same problem. Also if anyone knows how to fix the rear latch so you can open it that is broken as well. I need to open to fix and to get to my tail lights to replace them. If any one can help please do so soon.