If you are replacing an old version of a video with a new version and no longer want the old version in your account, the easiest way to replace the video in the collection is to use the "Replace Video" feature on the video within your dashboard (not within the collection editor).
To learn how to replace a standalone video, please read this article. Doing this will quickly swap the video out and all collections that referenced the old video will now reference the new one.
If you have already uploaded the new video without using the ‘Replace Video’ feature, you will want to start by adding the new video to the collection (using the Video Manager in the collection editor) so that both the old version and new version are associated with it. You can then go into the settings of each button that references the old video and select "Change Video". Simply select the new video version and click "Save". Once all buttons or scenes have been updated, the old video can be removed from the collection from the Video Manager.