You can slow down a video on an iPhone using the built‑in Photos app or the free iMovie app.

Using the Photos app

This is best if the video was recorded in Slo‑Mo, but newer iOS versions also let you adjust speed more generally.

1. Open Photos and tap the video you want.
2. Tap Edit (top right).
3. If it’s a Slo‑Mo video, use the bar of vertical lines under the timeline: drag the denser lines (slow motion area) wider to make more of the clip slow, or narrower to reduce it.
4. Tap Done to save.

Using iMovie

Use this if you want more control, or the video wasn’t shot in Slo‑Mo.

1. Open iMovie and tap Create Project → Movie.
2. Select your video and tap Create Movie.
3. Tap the clip in the timeline, then tap the Speed icon (speedometer).
4. Drag the slider to the left (toward the turtle) to slow the video down; you can go down to about 1/8 speed.
5. Tap Done, then use the Share button to save the edited video back to Photos.