Hello,
Simply find the following code in the Slider's HTML Code:
data-ytid="LXb3EKWsInQ" data-videoattributes="version=3&enablejsapi=1&html5=1&volume=100&hd=1&wmode=opaque&showinfo=0&rel=0;&start=08;"
This: data-ytid="LXb3EKWsInQ" refers to the Youtube ID which the Slider uses currently. However, if you are planning to use an MP4 Video, consider checking out this Official Guide: https://www.themepunch.com/revsliderjquery-doc/background-image-video/#html5-video-background .
This should definitely work fine. Hope this Helps!
Let us know if we can help you with anything else or if you find any further issues.