As previously stated, turn the secondary regulator all the way down (anti clockwise - i realise this feels odd) and it'll vent the vast majority of the contained air. To remove the secondary reg while the tank is rather full can be an arse but I found using a bench vice on the sides and turning the bottle itself gave me enough leverage to loosen it. It'll still be a bit stiff as the pin on the primary reg is still under pressure and you can't disengage it externally on the balystik.