input: 
  def replace_string(sentence: str, old_word: str, new_word: str) -> str:
      """Replace a given word in a sentence with another word.
      >>> replace_string("I love walking on the beach", "walking", "swimming")
      "I love swimming on the beach"
      >>> replace_string("Python programming is fun", "Python", "Java")
      "Java programming is fun"
      """
      new_sentence = sentence.replace(old_word, new_word)
      return new_sentence
  
output:     new_sentence = sentence.replace(old_word, new_word)
      return new_sentence