Close the garage door and locate the main power unit for your opener and follow the track towards the door. Near the door you should see a mechanism similar to the one in this picture with a rope or string attached and hanging down.
With the door still closed pull firmly on the rope to disengage the carrier from the track. You should feel it disengage. You can now open and close the door manually.
Depending on the type of automatic opener either the lever will spring back and re-engage automatically or you must manually pull the rope up towards the ceiling to put the lever back in place. Once the lever is in place you can operate the control to open the door with the automatic opener and the carrier will re-engage and open the door.