two manners:
1- place the arrow on the edge of the clip (it will change to a sort of square...) , ctrl+click'n'drag to left or right to stretch or compress
2- click on the event then go to "Insert" menu, then "Video Envelope" then "Velocity". A line will appear on you track: you "grab" it with the hand and go up or down to raise or lower speed. You can insert points by putting the hand on the nline again then doing shift+click. With these points, you can accelerate then slow down inside the same track.

Have fun!