input: Write a python function to replace words in a string and return the modified string.
  assert replace_words("Hello, how are you today? How is your day going?", "how", "what") == "Hello, what are you today? What is your day going?"
  assert replace_words("The quick brown fox jumps over the lazy dog", "fox", "cat") == "The quick brown cat jumps over the lazy dog"
  assert replace_words("It was the best of times, it was the worst of times", "times", "moments") == "It was the best of moments, it was the worst of moments"
output: def replace_words(input_string, old_word, new_word):
      return input_string.replace(old_word, new_word)