Select a piece of film in your Timeline with the Selection Tool.
Go to Modify > Change Speed.
The rate should be 100%.
If you change it to 50% your video will run at half speed.
If you change it to 100% your video will run at double speed.
Using the Start and End options will allow you to make the selection gradually slow down or speed up.
Experiment with them and see what results you get.
Click the Reverse option will make your video play from back to front.