input: 
  def replace_string(sentence: str, old_word: str, new_word: str) -> str:
      """ Replaces a given old word with a new word in a sentence and returns the new sentence.
      >>> replace_string("I love walking on the beach", "walking", "swimming")
      'I love swimming on the beach'
      >>> replace_string("She is reading a book", "book", "novel")
      'She is reading a novel'
      """
  
output:     return sentence.replace(old_word, new_word)