Open the Photos app
Select the video
Tap Edit
Tap the speaker icon to mute the audio
Tap Done
Save the video
Open iMovie
Create a new movie project
Import the video
Tap the video in the timeline
Tap the volume icon
Reduce the volume to zero
Tap Done
Export the video
Use a third-party video editor app
Import the video
Find the audio or volume setting
Mute or remove the audio track
Save or export the edited video