[kcsa75]

Get a hammer!

Seriously, it shouldn't be a big deal. You should probably have a helper. There may be a locking band on the outside that will need to be removed. It will be either a separate piece of rubber in a groove in the gasket that you can just find the end and pull out or there will be a ridge where one piece of the rubber overlaps the other. If it's the latter, find something that you can put between the two sides -- a cotter pin puller works best, but a screwdriver will work -- and go around the whole window, then you should be able to CAREFULLY push the window out. Get it started in a corner and CAREFULLY work your way around the glass. Once you get about half-way around it should come right out.
If you don't have either, from the inside just start working the rubber gasket loose while gently tapping the glass out. You should probably have your helper stationed in the bed to catch the glass once it's loose.
To put the slider in, wrap the rubber gasket around the sliding window, then get a piece of string and place it in the grove where the gasket goes over the flange on the cab. The string should go all the say around and both ends should meet in the middle on the bottom. Have somebody hold the glass up to the opening with the strings inside the cab. Then start working the gasket over the flange by pulling the string out while your helper on the outside GENTLY taps the outside of the glass. Once you get it started, it actually goes pretty fast and easy. If it's stubborn you may want to use some liquid dishsoap on the gasket, but I wouldn't use soap unless I have to as it can get to be a mess.
Good luck!