If you already have a YouTube Video setup on your Wix.com site and you would like to replace it:
1) Hover over the new video you want to add on You Tube and Right Click and select "Copy Video URL"
2) Log into your Wix Site as an editor
3) Navigate to the "Video Page"
4) Click on the video you want to change
5) Click on "Change Video"
6) Paste the Video URL you copied in Step 1 under "What's the Video's Web Address".