On either the new uploads page, or video or photoset library pages, click the Bulk edit button:

Select the videos you want to edit by checking the box at the top left of the card:

Or by selecting all videos showing on the page by checking the Select all box:

Once you have selected the content you want to bulk edit, click the Edit button:

You will then be presented with a page where you can select the properties you want to edit. Make your selection(s):

Note: Only existing properties that are common to all selected media will appear and be editable. For example, if all of your selected videos have the tag "Premium", then that tag will appear and can be removed from all selected videos. If only some of your selected videos have this tag, it will not show and the only option will be to add it to all selected videos.

If editing already existing content click the Done button. If editing new content, complete the remaining steps.