1) End plex
2) go to the 'Services' control panel of Windows and Stop the 'Plex Update Service'
3) copy this folder and all its content to the new location
C:\Users\<username>\AppData\Local\Plex Media Server
The copy process can take several hours if you have a fairly big library.
4) start plex
5) go to Settings - Server - General - Show Advanced - 'The path where local application data is stored'
6) type in the new location
Beware! The path you type in must not include 'Plex Media Server' as last part.
So if now your plex data reside in
G:\plexdata\Plex Media Server\ you type in only
into the input field. (Keep in mind that G: must use the NTFS file system, not ReFS which gets used with Windows Storage Spaces. It must also not be a networked drive. exFat is also not a viable option.)
7) save changes
8) end plex
9) restart computer
Once you verified, that everything is working (including fetching metadata for new items!) you can delete the old plex data directory.
Original writeup by Ottokerner https://forums.plex.tv/discussion/comment/851746/#Comment_851746