Mono/stereo problem with videos files. How to fix?

I have some video files which have stereo audio tracks that were created from a mono source. I think the mono signal got sent to the right channel and not the left because when I created the files in OBS I forgot to select mono audio recording. When I play the videos all the sound is on right side only. Hope that makes sense...

How can I either mix down/combine the stereo tracks intoto a single mono track or replace the empty left channel with the audio content of the right channel? Is there a way to do this without reencoding the whole video file?

I could do it in a video editing gui by copypasting the audio track to the other empty channel but I wonder if there might be a more efficient way of doing it. An ffmpeg command perhaps?

Many thanks