MP4 copy left audio track to right and save video

I found a question and answer here but it seems like only half an answer for someone as inexperienced as myself.

I have a lot of mp4 videos with audio that plays in only my left ear. I’m nearly deaf in my left ear and need to make the audio play in my right ear too.

I got to the copy left to right audio part but then what? The referenced answer leaves you hanging with no instructions for what to do next. Play the video to see if the audio was copied? Save the video, export it?

Thanks for the help.