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

This side can be slid off easily, since the bottom axis will come out with the armrest

Mind the two circular plastic washers on the lower axes

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

The second (parallel) washer on the side, where the first armrest was before, can be removed by pulling the shaft out a little, and then pushing it back (slightly tilted), thus pushing the plastic washer out

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)