Hi "dpb",
For help with editing Photos with your iPhone, try updating the iOS to the latest version first. The steps in the article here can assist: Update your iPhone or iPad - Apple Support
Moreover the steps for changing a section of slow motion can be found below and in the article here: Trim video length and adjust slow motion on iPhone - Apple Support
"Change the slow-motion section of a video shot in Slo-mo mode
- Open a video shot in Slo-mo mode, then tap Edit.
- Drag the white vertical bars beneath the frame viewer to set where the video is played in slow motion."
It also states "Note: A video saved as a new clip can’t be reverted to the original."
If you need further assistance, reach out to Apple Support directly.
Contact - Official Apple Support
Contact Apple for support and service - Apple Support
Best.