Open the Photos app
Select the video you want to edit
Tap Edit
Drag the start and end sliders to trim the video
Tap Done
Choose Save Video or Save Video as New Clip
To cut a video into parts, use iMovie or another video editing app
Open the editing app and import the video
Place the video on the timeline
Move the playhead to the cut point
Tap Split or Cut
Repeat for additional cut points
Delete unwanted sections
Export or save the edited video