On one type of gas bottle where the regulator fitted to a nozzle from the side of the neck and had a a circular stop cock valve on the top of the neck, you just took it outside with the regulator removed then turned the stop cock valve and it released the gas via the nozzle where the regulator would have been attached. Safe as long as the nozzle is facing away from you.
The other where there was no stop cock only the valve on top he used a small "valve pressure relief tool", I never did this as it was easier exchanging ours to the ones with valves on top, I like things simple like me!
Hope that explains things, I have never filled a gas bottle here but I would presume the people filling them would know about this.