Select the clip you intend to speed up or slow down and select edit. Navigate to the Speed Tab.
Adjust either end of the range to select the start time and end time of the effect.
Move the value nob to the desired speed: You can speed up and slow down the video by a maximum value of 10 time the original speed. If you want to make it slower move the nob to the left. If you wish to make it faster move the nob to the right.
Select the smooth in/out option if you want the video slow down and speed up gradually over 1 second instead of instantly.
Preview the effect a couple of times to see if you need to increase the value. If you are not happy with effect and want to remove it. Click the trash can symbol on the Speed Tab:
You'll notice that if you make your clip slower the overall length of the clip will be longer. If you speed up your clip the overall length will be shorter. Once you choose the effect you wish to keep, adjust the rest of your story if needed.