Remove the two screws from the other side of the chair (on the other, remaining armrest)

Remove the armrest similarly to the other side, along the screw axis

Mind the two circular plastic washers on the lower axes

Mind the two plastic inner washers (that are parallel to the axes

Finally, the shaft may be removed from the other armrest as well, however, it can be stuck sometimes (you could possibly use a plier, but it can damage the shaft's surface)