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 Slow-Mo video
Drag the vertical bars on the timeline to change the slow-motion section
Tap Done to save changes
To record a slow-motion video
Open the Camera app
Swipe to Slo-Mo mode
Tap the Record button
Stop recording when finished
Open the video in Photos to edit the slow-motion section if needed