Open the Photos app
Select the video you want to edit
Tap Edit
Tap the speed or adjustment option if available
Drag the speed slider to slow down the video
Tap Done to save the changes
If the video is a normal video, use iMovie or a third-party video editor to slow it down
Open iMovie
Import the video
Tap the video clip in the timeline
Tap the speed control
Move the slider left to reduce the speed
Tap Done to export the slow-motion video