I have serviced non-aviation fire extinguishers, and also those on helipads and helidecks offshore. A person with the knowledge and the permission can do this. The fire extinguishers for engine fires protection are a different beast. If found underweight on scheduled inspection, fired off or due hydrostatic test, these extinguishers have to go back to an approved service centre for maintenance, testing and refill. Other than the fire extinguisher removal itself and the removal of the explosive squibs there is little that can be done before sending off to the approved service centre.