Hey,
Right click the event you want to reverse and you will see insert/remove velocity envelope towards the bottom. select it and you will see a green line across the specified event. Now you can increase or decrease the speed of the event, up and down, with a left click( and drag) on the line. If you right click the point at the far left of the green line you can choose to reverse by 100% or just adjust it by percentages(if you want more or less than 100%).

hope this helps!
ciao-Jason