Here is an example of how you could modify the IP address of an HTTP header using C#:

using System;
using System.Net;

namespace ModifyIPAddress
{
    class Program
    {
        static void Main(string[] args)
        {
            // Create a new HTTPWebRequest object with the URL of the website you want to connect to
            var request = (HttpWebRequest)WebRequest.Create("http://www.google.com");

            // Set the IP address of the remote host
            request.ServicePoint.BindIPEndPointDelegate = delegate (ServicePoint servicePoint, IPEndPoint remoteEndPoint, int retryCount)
            {
                return new IPEndPoint(IPAddress.Parse("192.168.0.1"), 80);
            };

            // Send the request and retrieve the response
            var response = (HttpWebResponse)request.GetResponse();

            // Print the response
            Console.WriteLine(response.StatusCode);
        }
    }
}