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_to_replace = 2; // Replace this occurrence
    		var cnt = 0;
    		var result = Regex.Replace(s, "[^=]+(?=&)", m => { 
    			cnt++; return cnt == idx_to_replace ? "REPLACED" : m.Value; });
       		Console.WriteLine(result);
	}
}