None of these operations are destructive. That means they give you a new file, they don't write over the old one, so they're safe to try as long as you're not low on HDD space. If something goes wrong and it seems to be taking too long, just Ctrl+C and it'll stop.
ffmpeg -i #VIDEOFILE -codec copy -map 0:v:0 -map 0:a:#N output.mp4
Where #VIDEOFILE is your video's filename (quote it if it has spaces in it) and #N is the index of the audio track you want to keep, starting with 0 for the first audio track (so if you have 3 tracks, they're 0, 1 and 2).