You do it yourself. Purchase a "rearming kit" and have one hand as a spare. I would also recommend you put it on, adjust it correctly, and jump into some water [lake, river, pond, pool] to see how the PFD reacts. It is very different from the regular PFDs. I'm glad I know what to expect if I do have to abandon ship.......
After you test it out, you will need to install the rearming kit. And get a new spare one.