Open the Photos app
Select the video
Tap Edit
Tap the speed icon if available
Drag the speed slider to the left to slow the video
Tap Done
If the video was recorded in Slo-Mo, open it in Photos
Tap Edit
Adjust the white bars at the bottom to extend the slow-motion section
Tap Done
If no speed option appears, use iMovie
Open iMovie
Create a new project
Import the video
Tap the video clip in the timeline
Tap the speed control
Drag the slider left to slow it down
Tap Done and save the video