The Handyman's Pocket Book: Re-Fixing Knife Handles.

To remove the blade punch out the rivet (if any) and put the handle into hot water to melt the old resin.  If the handle is of composition, however, keep the heat moderate as otherwise it may crack or warp.  Refix the blade with molten resin and plaster of paris.  Heat the resin in a tin and shake plaster into it and stir.  Pour it into the hole in the handle. Heat the tang, press it into the hole, and leave to set.  Alternatively use one of the special 2-part resins such as Araldite.