If the velocity is constant, the displacement graph would have a straight line, instead of a curve. The line is a curve because the displacement is different for the same amount of time during the recorded period. As you can see, the line tend to curve more at the end, which means the velocity has increased comparing to the beginning.
Plus, if you look at the actual numbers, for the first 6 seconds, it travelled 36m, but for the next 6 seconds, it travelled 144/2 = 72m, which is not the same as 36m, so we can spot the change in velocity.
