Use JSTool

As of Notepad++ v7.6, you can use the Plugin Admin option in Notepad++ to install JSTool.

Manual Install

1. Download it from https://www.sunjw.us/jstool/npp/#download
2. Copy JSMinNpp.dll to the plugin directory of Notepad++. You may need to create a directory named "JSMinNPP" in the plugin directory after version 7.6.3

Here's an example of JSON that's been formatted using the tool:

{
  "menu" : {
    "id" : "file",
    "value" : "File",
    "popup" : {
      "menuitem" : [{
      "value" : "New",
          "onclick" : "CreateNewDoc()"
        }, {
          "value" : "Open",
          "onclick" : "OpenDoc()"
        }, {
          "value" : "Close",
          "onclick" : "CloseDoc()"
        }
      ]
    }
  }
}

Tip: Select the code you want to reformat, then Plugins | JSTool | JSFormat.

You might be able to use a JSON formatter to accomplish the task (since it would likely complain or do something wrong -- say, like indent improperly -- if the JSON's invalid). See Is there an extension to reindent JSON in Notepad++?. Similarly, there's a JSONViewer Notepad++ plugin the might help in the same way.

Another less-than-ideal alternative would be to just use one of the online JSON validators.