Let the mitre arm saw blade come to a complete stop before attempting to release it. Pull out the lock pin on the turntable below the saw and raise the saw all the way up.
Unscrew the mitre lock handle one-half turn counterclockwise to allow it to move and push down on the mitre lock plate with your thumb. Hold the lock plate down and move the mitre saw arm to the desired angle.
Lift your thumb off the mitre lock plate and turn the mitre lock handle clockwise to lock the arm in place.