There is hopefully a valve isolating the tank from the system.
Turn off the power to the boiler.
Turn off the isolating valve, then unscrew the tank until you see water begin to dribble. If the system is going to be operated at a nominal 12 psi, pump up the air side of the tank to 12 psi minimum , 15 psi maximum. That should force more water to dribble as the air pressure pushes diaphram into the water side of the tank.
Give it a few minutes for the pressure to stabilize as the water is pushed out, or unscrew it another 1/4 turn to speed things up. Recheck the charge pressure and adjust as-needed. Then screw the tank back on tight, and open the valve to the system before re-starting the boiler.