In the current version of Powerpoint (Powerpoint 2021, version 16.0), this is not possible. The only way to replace this is to delete the old one, insert a new one, then define the settings for that video again.

Looking at the replies at the MS forum, this volunteer says it's the simplest solution. It can also be done using Visual Basic, but that's clearly not for the people who have never used VB. So it's basically the only solution to delete it, then add the new one. https://answers.microsoft.com/en-us/msoffice/forum/all/how-do-i-replace-one-video-with-another-video-in/49878e93-4d72-47a7-8f53-a17d1b4739e9