using System;

using System.Linq;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        string json = @"

            {

                ""data"": [

                    {

                        ""A"": ""1"",

                        ""B"": ""4""

                    },

                    {

                        ""A"": ""2"",

                        ""B"": ""5""

                    },

                    {

                        ""A"": ""3"",

                        ""B"": ""6""

                    }

                ]

            }";

        JObject obj = JObject.Parse(json);

        obj["data"] = new JObject(obj["data"]

            .Children<JObject>()

            .SelectMany(jo => jo.Properties())

            .GroupBy(jp => jp.Name)

            .Select(g => new JProperty(g.Key, new JArray(g.Values()))));

        Console.WriteLine(obj.ToString());

    }

}