Right click on the event and select "Reverse." Be sure to click on both video and audio events if that's what you want, or you can do picture and sound separately.
That is the Mixup. Right click reverse is only available in V5. Audio reverse is not available in V4 at all but you can reverse picture by inserting a velocity envelope (right click on event and select Insert/Remove Envelope/ Velocity) and pulling the green envelope line down into negative numbers.