Open the Photos app
Select the video you want to edit
Tap Edit
Tap the speed control if available
Drag the speed slider to the right to speed up the video
Tap Done to save the changes
If the speed option is not available, use iMovie
Open iMovie and create a new project
Import the video into the project
Tap the video clip in the timeline
Tap the speed icon
Drag the speed slider to the right
Tap Done to save the sped-up video
If needed, use a third-party video editor app from the App Store