You should be able to change it manually if you go to Control Panel > All Control Panel Items > Default Programs. There you’ll see different options, the one you want is “Associate a file type or protocol with a program”, once you’re there just look for the extension .json and associate it with Notepad ++.
If this somehow doesn’t work still, there’s always the option to do it through the registry with regedit. The way to do this would be by running regedit and once you are in the Registry Editor go to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.cpp
there you will find all the file extensions and their configuration as to how they are being open. You could test if deleting the key, which would appear as a folder, of “.json” helps with this issue.