.entities files are compressed and must be decompressed in order to edit Entities.
EntityHero by Scorp0rX0r - Download
File -> Export to text (make sure the file extension is still .entities)
SAMUEL Asset Extraction Tool by SamPT - Download
Currently exports textures, .decls, and .entities files.
idFileDecompressor by Zwip-Zap Zapony - Coming Later
Even though the .entities file is decompressed, make sure it still retains the .entities file extension.
When editing a decompressed .entities file while you are in game, you will need to create a folder called "overrides" in the DOOM Eternal installation directory. Structure it like this:
> This is the level file path for Hell on Earth.
You can place as many level files you like in the overrides folder
With M347h00k installed, you can enter your level and edit the decompressed entities file while you are in-game.
Typing in the console command,
mh_force_reload will reload the level with the recent changes you made in the text file.