Your upholstered furniture may be equipped with an optional manually adjustable armrest. The angle of the armrest can then be continuously adjusted up and down:
to adjust the armrest, grip it with both hands and pull it upwards. The armrest locks in the position that you leave it in. The armrest CANNOT be pushed back down from a half-way position! First, pull it all the way up. Only then will the built-in lock release and allow you to push the armrest back down to its starting position.