using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
    		var s = "=StringOne&=StringTwo&=StringThree&=StringFour&";
    		var idx = 2;     // Replace this occurrence
		var result = Regex.Replace(s, $@"^(=(?:[^=&]+&=){{{idx-1}}})[^=&]+", "${1}REPLACED");
       		Console.WriteLine(result);
	}
}