How do you refill co2 tank?

Answer 1

If you are not trained on filling Co2 tanks (unlike HPA tanks) you should not attempt this, on risk of breaking the burst disks, or freezing your hand. An HPA tank however, you can simply hook up and fill.

Answer 2

Most likely you will have to go to a paintball store or field to get it filled. Otherwise, screw in the top of the tank into the filler. Chill the CO2 tank and turn on the large tank to fill. Weigh to desiered weight.