On Windows, Plex keeps (or used to keep) prior versions in an Updates directory.
Look in the Plex Server install directory for the Updates
folder (c:\Program Files…).
Alternately, look in %LOCALAPPDATA%\Plex Media Server\Updates
(ex: c:\users\username\AppData\Local). AppData is normally hidden. Enable viewing of hidden files in Windows Explorer to view the contents (In Windows Explorer, click View tab, then the checkbox for Hidden Items).
I have not run Plex on Windows in awhile, so cannot say for certain the Updates folder is still present.