The particles have maximum upwards velocity at the position where the displacement is changing most rapidly, which corresponds to the crest of the wave.
The particles along the rope will have maximum upwards velocity at the position where the displacement is changing most rapidly. In the given graph, this corresponds to the position where the slope of the graph is steepest. The steepest slope occurs at the crest of the wave, where the displacement changes from positive to negative.
Therefore, at the distance along the rope where the graph shows the displacement changing from positive to negative, the particles have maximum upwards velocity.