Removing (insane amounts of) old data?

[OttoKerner]

By shutting down Plex server, I guess.

P.S.
Reads this how-to, particularly the “Addendum” part:

[HowTo] An extended guide on how to move the Plex data folder on Windows

By default, Plex server stores all its own data on the system drive, in the AppData path of the Windows user. You might want to change that location, if your system drive is a small SSD or you are simply having capacity issues. Moving Plex data to a dedicated drive also makes it easier to retain your Plex server data when changing machines or re-installing Windows from scratch. For larger servers/collections, I recommend a dedicated, internal SSD drive. Exit/End/Quit/Close plex, using the t…