On mine, there is a zipper toward the swivel end on the bottom. To remove the arm, turn it to the up position (counter-clockwise) then push inward and turn CCW some more. then pull out . You may have to turn it back and forth a bit to find the removal slot.
I took my broken one,(looks like someone sat on the outer end) apart and tried to glue it back in shape, only lasted a little while. I think I will try calling the seat maker,to get the wooden part replaced