In the Preview section of the app, you’ll find a ‘Manually Adjust Timing’ button. Click that button.
Once that button has been clicked, you’ll see a timeline appear – if you have a voiceover, you’ll see the voice in a wave form here.
To change the timing of your videos you can click and drag the green vertical bars the arrows pointing downwards are showing. Moving the green vertical bar to the left will mean the slide comes in earlier and moving the bar to the right means the slide will come in later.
You can also click and drag the end of the video out extend the video or make it shorter