Center punch the rivet. Drill it out with a very small drill bit, then re-drill with consecutively larger sizes until you cut the rivet head off completely. Drill slowly and keep the rivet soaking with with water to keep it cool enough not to melt the plastic. Replace with a small machine screw, and nut from behind, assuming you have some access to inside the handle tube. Getting a wrench inside the tube is not really necessary, once you catch the nut with the screw, you should be able to tighten it completely.
