You can use iMovie to convert HDR to SDR easily. Here's how.
Step 1. Find iMovie on your iOS device.
Step 2. Add HDR videos filmed through your iPhone to iMovie.
Note: While sharing a movie, you can click Options and turn on HDR to output the film in HDR.
Step 3. Turn off the HDR button to save your video's SDR version to get your video in SDR format.