Replace x with y

Write a method that replaces every letter 'y' in the string with 'x'. Assume that string contains only lower case letters.
Expected input and output
ReplaceXWithY("yellow") → "xellow" ReplaceXWithY("mushroom") → "mushroom"
using System;

namespace CSharpExercises.Exercises.Library_functions
{
    class ReplaceXWithYTask
    {
        public static string ReplaceXWithY(string word)
        {
            string[] words = word.Split(' ');

            for (int i = 0; i < words.Length; i++)
            {
                if (words[i].Contains("y"))
                {
                    words[i] = words[i].Replace("y", "x");
                }
            }

            return String.Join(" ", words);
        }

        public static void Main()
        {
            Console.WriteLine(ReplaceXWithY("yyy"));                                   // xxx
            Console.WriteLine(ReplaceXWithY("strawberry youghurt"));                   // strawberrx xoughurt
            Console.WriteLine(ReplaceXWithY("tym ryhosx oifg 6 t6 ypeg ergh"));        // txm rxhosx oifg 6 t6 xpeg ergh
            Console.WriteLine(ReplaceXWithY(""));                                      // /empty string/
        }
    }
}