using System;

using System.Web;

using System.Collections.Specialized;

public class Program

{

    public static void Main()

    {

        Uri originalUri = new Uri("https://software.apextoolgroup.com/version-check?t=400&csw=S168813-1.9.2-std&csn=WH-C71234&mcsw=S168025-614&mcsn=4A00123ssw=STMD_H_3.1.0.4.4&ssn=SN81234&tsn=abcdefg&o=screen");

        NameValueCollection queryParams = HttpUtility.ParseQueryString(originalUri.Query);

        queryParams["o"] = "sticker";

        Uri updatedUri = new UriBuilder(originalUri) {Query = queryParams.ToString()}.Uri;

        Console.WriteLine(updatedUri.ToString());

    }

}