Open the Photos app
Select the video you want to shorten
Tap Edit
Drag the left or right trim handles to choose the part you want to keep
Tap Done
Tap Save Video or Save Video as New Clip
Use iMovie if you need more advanced trimming
Open iMovie and create a new project
Import the video
Tap the video in the timeline
Drag the trim handles to shorten it
Tap Done
Export the edited video to Photos