Open the Photos app
Select the video you want to edit
Tap Edit
Tap the speed icon or adjust the video if it is a Slo-mo clip
Drag the white sliders to choose the slow-motion section
Tap Done to save changes
If the video is not already a Slo-mo video, use iMovie or a third-party app to slow it down
In iMovie, import the video
Tap the clip in the timeline
Tap the speed control
Drag the speed slider to the left
Tap Done and save the video