input: Write a function to replace a given string with another string. If the given string is found in the provided sentence, replace it with the new string. If the string is not found, return the original sentence unchanged.
  
  def replace_words(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)
  
  assert replace_words("I love walking on the beach", "walking", "swimming") == "I love swimming on the beach"
  assert replace_words("I love reading books", "books", "mangas") == "I love reading mangas"
  assert replace_words("The quick brown fox", "jumps", "leaps") == "The quick brown fox"
  assert replace_words("Winter is coming", "coming", "far away") == "Winter is far away"
  assert replace_words("", "old", "new") == ""
output: def replace_words(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)