Sorry, wrong. VP doesn't have that functionality. But you can suggest it... https://www.nch.com.au/suggestions/index.html?software=VideoPad
That said, right-click the audio clip and Edit With Wavepad (when available). WP displays the file. Click Stereo at the bottom of the window and convert to mono.
Exit WP with the X at the top-right. The file will be placed on the VP timeline. You can then unlink (if necessary), delete the original and slip the video into place.
To return to stereo, find the mono file in the PC's MUSIC folder and use Wavepad to make it stereo. Then import into VP.
Here's the free version of Wavepad, which may or may not provide the conversion feature. Check it out.