You have to use the plugin manager of Notepad++ and search for the JSON plugin. There you can easily install it.

This answer explains it pretty good: How to reformat JSON in Notepad++?

Here are the steps that worked for me:

1. Download the plugin and extract the plugin dll file.
2. Place the plugin.dll file under plugin folder of notepad++ installation. For me it was : C:\Program Files\Notepad++\plugins
3. Start Notepad++ as an elevated administrator and then go to: Settings -> Import -> Import plugin(s)... (import the plugin).
4. Notepad++ will show the restart message. / Sometimes it may not show it.
5. Restart the notepad++.
6. Should see new plugin under the Plugins menu. ALL DONE!!

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.