Videopad doesn't alter the original, so it can be loaded once more to return to how it looked at the outset.
After loading, right-click the clip and choose Change Aspect Ratio. Test the many options, to see which is best. Try Stretch. Then drop the clip onto the timeline.
If the video is on the timeline, as yet unchanged, try a few video effects. Letterbox to Aspect Ratio and Stretch to Aspect Ratio are good candidates.
Depending upon the A/R of the original apply the Scale effect. Some picture content may be lost.
If you share the video, someone may have further suggestions. Here's how...
Upload the file to a free server, such as Google Drive or MS OneDrive, right-click the link and Share or Copy. Copy the result and paste it here or to me in a Private Message via the mail envelope in the top-right corner of this forum. When using Google Drive, right-click the file, then click SHARE | SHARE and change "restricted" to "anyone with link can view.**