JSon code shouldn't need to be opened, calibre will read the file directly for anything it needs to do. File associations like you are thinking of will only be used to determine what program to manually open and edit said file. And it is a bad idea to try editing any of that stuff yourself, unless you REALLY know what you are doing.
JSon is not an ebook filetype, so you don't need to touch it anyway. I assume you are speaking of the metadata_db_prefs_backup.json file? calibre uses that as a backup of database preferences, and will handle it all by itself.
Nevertheless, if you are determined to open it, you can manually choose how to open any file; directions are here:
http://windows.microsoft.com/en-us/w...a-type-of-file. Open it in Notepad or any other Text Editor.
As a general rule, though, NEVER open or move or edit anything in the Calibre Library folder, since it may severely mess up your library. calibre expects to be solely in charge of anything you do there, and if you want to use your ebooks, you should let calibre handle it. Read an ebook? Fire up calibre, locate the library record, and click view. Transfer an ebook to your device? Use calibre send to device. Edit the metadata? Tell the new metadata to calibre. That's what it's there for, and it really does do the best job anyway.