How do I refill a fire extinguisher or should I just buy a new one?
I used to be a fire extinguisher repair tech.  This unit should not be refilled even though it is possible with the right equipment.  The plastic/nylon heads will develop cracks and leaks over time.  Buy a new unit with a metal head.  Those can be refilled and will last you a long time. Amerex, Badger, General, Ansul are all decent brands.