I really depends on your software.
Video software is generally pretty bad for editing audio, but if you have a mixer function in your software you can use it to turn stereo into mono.
What I do: export the audio into an audio app (there are many out there–some free), edit the audio (in this case, splitting the stereo into two mono tracks) put the audio on both tracks and reimport it to your video app.
Another thing you can try, depending on the noise, is some sort of filter once you have it in an audio app. If it is hiss, you can roll off the treble. Filters have a way of sounding cheesy, but EQ sometimes does the trick, especially for rumble.
If you are using a high end video app, you can filter the sound in place.
Finally, you can rerecord the audio; depending on the type of material, that may not be practical.