Reshafting requires removing the old one and installing a new one. Assuming it's intact, the old shaft can be removed by heating the hosel with a heat gun, or a propane torch if you're really careful. You need to heat the epoxy holding the old shaft to its failure temperature, which is typically somewhat above the boiling point of water. It works best if you can keep a constant force and/or torque on the shaft while heating, so the failure will be observed immediately. If the shaft is broken off, you might be able to grip it with locking pliers. If it's flush with the top of the hosel, something similar to a screw extractor can be used to get a grip on it. Once the old shaft is out, clean up the inside of the hosel. I use sandpaper rolled into a cylinder, and work until it's shiny. Installing the new shaft requires cutting it to length, roughening the tip with coarse sandpaper, coating the tip and the inside of the hosel with suitable epoxy, and assembling. It's usually recommended to knock some bumps into a metal shaft tip, to give the epoxy an uneven surface to hang onto. Graphite shafts need to have the outer coating removed first, since the epoxy won't stick to it. Most heads also require a ferrule, which is installed on the shaft before inserting it into the head. I won't go into the details of shaft trimming; it's too big of a topic. I recommend Dave Tutelman's club design pages to help you decide how long to make it; you'll also need the trimming instructions for your particular shaft.