Download the latest version of Plex Media Server for Windows from here: https://www.plex.tv/media-server-downloads/#plex-media-server
Stop Plex server using its task tray icon (if present)
Uninstall Plex server using the regular Windows - Apps control panel
Install the downloaded server version.
Run it.
open your web browser and point it to http://127.0.0.1:32400
If it returns a message that says something about “currently performing database updates…”, let it sit until it finishes.
You can refresh the web browser from timew to time to see if something changed. But if your database is rather large, this process can potentially take hours.
Once the message returned does not mentioning the “database upgrades” anymore, you can continue here: Unable to access server after password reset - #116 by OttoKerner
