Login to your BigCommerce Site and go to Settings > File access (WebDAV). Once done, kindly follow the article below on how to use WebDAV.
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Support Article for WebDAV: Click here!
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Once you are connected to your store's server, you can now upload the video you would like to use. Please note that the video must be uploaded inside the content folder.
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Next, is to copy the name and file type of the video you've uploaded in the content folder. Lastly, go to the Theme Customizer and proceed to Mega Carousel > Background Video.
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​Sample Video URL: /content/videoname.mp4
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