All you need to do is open the valve and let some of the steam/water out. When the water starts to flow, turn the valve back off.
When they need bled, it is a sign of too much pressure in the pipes, the heat cannot circulate because it is being blocked. this should be done once a year, at the beginning of the heating season. If you have a company that checks your furnace and/or water heater, ask the technician to show you how to do it.