Hello guys i need help with how to replace the rear backing brake plates and hard is it 2 do

Let me guess, ole man'* winter road salt ate them away.
It is a lot easier to deal with this on a lift. At home, it could still be done on stands.
Envoy/Trailblazer-this video shows you what mainly is involved.
Would recommend for better salt protection, that the new backing plates receive a few coats of additional rust paint for longevity.
Using a power washer also helps for the back braking plates when you can, even though winter.

Thanx for the help i have to find a way the take out the 5/16" bolt inside the differential cuz the bolt was stripped before i even touched it

Probably going to need a quality extractor, but can you post a photo of the bolt to see how bad it is..

a set of 1/4" turbo sockets works well