GoPro cameras can record at high framerates (though lower resolutions). However, simple/free video editors are often incapable of slowing down or managing high framerate video. To deal with these clips, I use
ffmpeg, an open source, cross platform, command line tool that can convert to and from every video format you will ever encounter. The basic command is
ffmpeg -i <input> -filter:v "setpts:4.0*PTS" -r 30 <output>, where
<output> are the input and output files.
4.0 slows the video down by a factor of four, appropriate for 120fps video.
30 refers to the output framerate. (Without this flag, the video would output 120fps, but the output would look the same because every frame would be repeated four times.) This command does not affect the audio. There are other parameters that can adjust the sound, but I have not needed to deal with it.