Find where you want to end your speed change. Split the clip again. Select the section you want to speed change. Select the speed from the slide rule. You'll see it grow or shrink in the timeline.
Effects Adding effects is easy.
There are over half a dozen included, and you'll find them in the Effects pane. To apply them, first select the clip you want to alter and pick an effect from the list. You should preview the clip before you commit to the edit. Effects can also be a good way to fix video that is too dark or too bright, or to convert color video to black and white. Head over to the iMovie website to purchase more special effects programs.