Probably by calling out a fridge repair company. :-?
They have the kit and the right sort of refrigerant gas to do it.
Thing is if it's lost refrigerant gas there is likely to be a tiny leak somewhere.
I had a fridge regassed some time back and it was OK for a while but then didn't do the job properly. So lesson learned there for me.
Fridges are supposed to last a 'lifetime' without needing a regas. My old Mum had a fridge which was over 30 years old and still going strong before it was taken away by the house clearance people.
I've just bought myself a nearly new larder fridge off Gumtree for �ã20, although I saw it running before I handed over the cash.