How do i replace the key and tumbler on a 91 chevy 1500?

You need to go to the part store and buy a special tool that compresses the spring inside the steering column. Don't cut corners, Disconnect the truck's battery. Without the tool you will not get the column back together. With the tool you will compress the spring, remove the clip above that dust shield. and then start to pull it apart. Remember to put everything back together how it was. That dust-cover looking thing must go back exactly how you found it or you will damage the plastic pieces to the horn circuit. When you pull the cover off for the horn be careful with the wire. Push and turn counterclockwise to remove. And you will pull out the directional lever and unscrew the little Phillips head screw from the hazard button so the turn signal Assembly can be moved so you can access the screw that locks the key cylinder in place.