How to rotate a video file?

You may have done this

$ ffmpeg -vfilter "rotate=90" -i input.mp4 output.mp4

but you would have

unrecognized option 'vfilters'.

Error splitting the argument list: Option not found

So you should run:

$ ffmpeg -i input.mp4 -vf "transpose=1" output.mp4

then input.mp4 will be rotated at 90 degrees to output.mp4.