The Default Library is created when you install MC, and is always (on Windows) found in the location "C:\Users\UserID\AppData\Roaming\J River\Media Center 23\Library". You can't change that. If you moved that library, or deleted it, there simply wouldn't be a Default Library anymore. That may cause problems, or MC may just create a new Default Library. I've never tried doing that.
You can, however, make the Default Library your regular library, if you have another library that you have been using in another location. Just backup your regular library, switch to the Default Library, and then restore the backup you just made. Of course, you can't do that when you are using a Client library connected to a Server. It wouldn't make sense to even try. The default library is and must always be a local library.
When a PC is used as a Client, it has a copy of the Server library, and that is stored in a different location. Specifically, "C:\Users\UserID\AppData\Roaming\J River\Media Center 23\Connected Library".
There is some information in the Wiki:
https://wiki.jriver.com/index.php/Library_Manager