1. Enable “Time Stretch”
Right-click on the layer Time → Time → Stretch 
2. Edit
To calculate the percentage you need to use the formula 100 / x where x is times slower. Lets say you need to make the video 3 times slower, 100 / 3 = 33.3. If you just need to make it 20% slower, you can deduct 20 from 100 and enter 80.
If you need to speed up the video, use the formulate 100 * x where x is time faster. Lets say you need it two times faster, 100 * 2 = 200.
You can also click and move to the right or to the left. 
