Yeah, the manual process to perform an edit can be troublesome for sure.
The appropriate Tag to modify is “Release Type”:
As you can see from the image above, the Release Type Tag accepts multiple values. Plex will use programmed logic to determine which value to focus on. The pictured example will be classified as a Soundtrack.
In order to modify the Release Type Tag, a specialized tag editor program is required. Most influential tags will not appear within an Operating System properties editor, the Release Type Tag included.
I recommend using MusicBrainz Picard (docs).
The MusicBrainz platform is truly useful and Picard is a wonderfully made program. Despite the negativity I threw at MusicBrainz in previous posts, the solution is of the highest quality and its leadership cares about developing a tool that helps people. With that said, there will be hiccups if you dive into editing as some fields are still near impossible to modify without using the voting system. Even Picard will reject the local modification of certain values, such as the Length Tag.
Here is a link to the MusicBrainz Picard Quick Start documentation.
Don’t be nervous, the Picard editing process is streamlined and more simplified than others. Just be sure to backup the files you are editing to avoid problematic and irreversible modifications. The most common problem I experienced is Picard identifying my release as another “version”, introducing differences in track ordering and Length Tag values.
Good luck and be sure to let us know how it goes! 
