Hi
AFAIK you can't create a gradual speed change from say 100% to 300% in a single clip. You can only change the speed of the clip as a whole.
If you just want to change the speed of a section of a clip....
Split the clip where the speed change is to start
Split the clip where the speed change is to end
Select (right click) the segment between the splits and Change Clip Speed from the menu The section will then show the speed alteration.
As mentioned there is no control to change the speed gradually, it is all or nothing.
You could however split a clip multiple times and increase/decrease the speed of each segment by a small even amount and this would achieve a progressive speed change.
Nat