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;                // Counter
    		var result = string.Empty;  // Final result variable
    		var rx = "[^=]+(?=&)";      // Pattern
    		for (var m=Regex.Match(s, rx); m.Success; m = m.NextMatch())
    		{
    			cnt++;
    			if (cnt == idx_to_replace) {
    				result = $"{s.Substring(0, m.Index)}REPLACED{s.Substring(m.Index+m.Length)}";
    				break;
    			}
    		}
       		Console.WriteLine(result);
	}
}