You can take your fire extinguisher to Continental Fire and Safety
Throw away small (18 inches or less) cylinder fire extinguishers, which must be empty, in your household trash. To empty, put the extinguisher in two heavy-duty plastic bags, such as contractor bags. Then, fold over the top of the bags to seal them and step on the seal with your foot to be sure it stays closed. Grab the lever through the bag with your hand and squeeze slowly to let out all the contents. Then tie the bags tightly and throw it out in your trash.
Dispose of aerosol extinguishers in the household trash. Aerosol extinguishers differ from traditional cylinder fire extinguishers---the fire suppressant is in an aerosol can, much like hair spray and spray paint.