Take it to a shop! if you want to do it yourself the normal way is to put the bottle on scales and fill it from a larger bottle to a fill weight. To be honest though CO2 is well out of date. A smart move would be to move onto compressed air.
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If you have a refillable CO2 tank it can be refilled by taking it to your local sporting goods store and asking them to refill it. It should be noted though that not all CO2 tanks can be refilled and so you should first ensure that yours can be.
With a paintball field CO2 filling station, DO NOT ATTEMPT TO DO ON YOUR OWN!! IF YOU DIDN'T KNOW WHERE TO FILL A TANK UP NEVER FILL ON YOUR OWN!!!
if you're asking how to refill the tank you take it to a local paintball shop or sporting goods store, they can fill them