Trim video length and adjust slow motion on iPhone
In the Photos app , you can trim a video you recorded on your iPhone to change where it starts and stops. You can also adjust the portion of a video that appears in slow motion when you record in Slo-mo mode.
Trim a video
In Photos, open the video, then tap Edit.
Drag either end of the frame viewer below the video to change the start and stop times, then tap Done.
Tap Save Video to save only the trimmed video, or Save Video as New Clip to save both versions of the video.
To undo the trim after you save, open the video, tap Edit, then tap Revert.
Note: A video saved as a new clip can’t be reverted to the original.
Change the slow-motion section of a video shot in Slo-mo mode
Open a video shot in Slo-mo mode, then tap Edit.
Drag the white vertical bars beneath the frame viewer to set where the video is played in slow motion.