iMovie
Change the connected clip effect in iMovie on Mac
You can freely change the effect used by a connected clip.
In the iMovie app on your Mac, select a connected clip in the timeline.
If the video overlay controls aren’t shown, click the Video Overlay Settings button.
Click the pop-up menu on the left and choose the effect you want to apply.
To apply the change, click the Apply button in the controls for the effect you chose.