Here is some quick and dirty video editing that one can use to for example submit app preview to app store or meet constraints on video like duration and resolution.
If we want to rescale our video we could do something like
ffmpeg -i preview.mov -vf scale=886:1920 video_640.mp4
if we want to strip sound we could
ffmpeg -i video_640.mp4 -an -vcodec copy output_file.mp4
if we want to add sound we could
ffmpeg -i output_file.mp4 -f lavfi -i anullsrc=channel_layout=stereo:sample_rate=44100 \\n-c:v copy -shortest output.mp4
if we want to change FPS we could do following
ffmpeg -y -i output.mp4 -c copy -f h264 output.h264
ffmpeg -y -r 30 -i output.h264 -c copy output3.mp4
now that we changed FPS video might be long
lets speed it up lets say 4 times
ffmpeg -i output3.mp4 -filter:v “setpts=0.20*PTS” output4.mp4
It takes a few google searches but here it is sublimed.