Not an easy job to replace. I bought a rear window seal and had a local glass shop replace it. They charged me $60 to do the labor.
To do it yourself first grind away the rusted spots and treat them with a rust stop agent. Find at a body shop supply house. If rust is deep enough should be filled with bondo after treatment and sanded out level.
To reinstall the glass, you put the rubber seal around the window and then tuck a string cord into the channel in the rubber, use a little heavy grease to hold it in place Press the window and seal against the channel and start the rubber onto the ridge. Then carefully pull the cord from inside the car to get the lip to slide over and onto the ridge and set into place. If done right it goes fast. Don't know about any seal, the grease usually helps the rubber slide into place and not bind and thus leak.