Replace two words

Given a string in which two words are separated by a char, write a method that replaces these two words.
Expected input and output
ReplaceWords("abc_xyz", '_') → xyz_abc ReplaceWords("trolling.master", '.') → master.trolling
using System;

namespace CSharpExercises.Exercises.Loops
{
    class ReplaceWordsTask
    {
        public static string ReplaceWords(string word, char ch)
        {
            string firstWord = string.Empty;
            string secondWord = string.Empty;

            for (int i = 0; i <= word.Length - 1; i++)
            {
                if (word[i] != ch)
                {
                    secondWord += word[i];
                }
                else
                {
                    for (int j = i + 1; j <= word.Length - 1; j++)
                    {
                        firstWord += word[j];
                    }
                    break;
                }
            }

            return firstWord + ch + secondWord;
        }

        static void Main(string[] args)
        {
            Console.WriteLine(ReplaceWords("dog_octopus", '_'));     // octopus_dog
            Console.WriteLine(ReplaceWords("a.b", '.'));                     // b.a
            Console.WriteLine(ReplaceWords("good very", ' '));         // very good
        }
    }
}