The Photos app on your iPhone, iPad, or iPod touch lets you change the length of your video or slow it down directly from your device.
A very common problem is having a video that's just too long, and file size too large to send through Mail or Messages. With the Photos app, you can change the start and stop time to make your video shorter.
To trim a video on your iOS device
Open the Photos app and tap the video that you want to edit.
Move the sliders on both sides of the video timeline to change the start and stop times. To preview your trimmed video, tap the play button.
Tap Done, then tap Save Video or Save Video as New Clip.
The Save Video saves only the trimmed version. Save as New Clip keeps the original version and saves an additional trimmed version in your library.
To undo a trim on your iOS device
On your iPhone, iPad, or iPod touch: Open the video that you trimmed. Tap Edit. Tap Revert, then tap Revert to Original. Note: If you're editing a video that you created using the Save Video as New Clip function, tapping Revert to Original will undo any edits that you've made, such as adjusting exposure or adding a filter. But it won't restore the video to its original length.