This is how you install it:
Find your installation location for Unreal Engine. Go into your version folder, i.e. "UE_5.3". Go into Engine/Plugins/ and then you know how there's the one big folder called "EditorDiscordRichPresence" inside the version folder provided within this download? If you still aren't sure, go into the folder. One of the files you'll notice is a ".uplugin". That's how you know that folder is the whole plugin.
Drag and drop "EditorDiscordRichPresence" into that Plugins folder, and then you can see the plugin in your plugins window without restarting UE. Hope this helped, it's super simple!