Hi,

To speed up a part of a clip, put a cut (trim - shortcut T) mark on either side of the part that you want to change and then apply the effect to this part. If you want a variable speed over time, then you need to also use keyframes. If you don't know how to use keyframes, look it up. Keyframes are used for almost every effect that one wants to vary. There are also tutorials that show how to use keyframes under the Tutorials tab and this tutorial about changing the speed.