ffmpeg -i input.mkv -r 60 -vf "setpts=PTS/60" -an output.mp4

• -i input.mkv input file
• -r 60 frame rate
• -vf "setpts=PTS/60" video filter, set presentation timespamp
• -an no audio (unless you speed up audio too, the output's length will stay the same)
• output.mp4 output file and format