input: Write a function to replace a given string in a sentence with another string. The function should return the modified sentence. If the old string does not exist in the sentence, the function should return the original sentence unchanged.
  def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)
  
  assert replace_word("I love walking on the beach", "walking", "swimming") == "I love swimming on the beach"
  assert replace_word("I love walking on the beach", "cycling", "swimming") == "I love walking on the beach"
  assert replace_word("The cat is sleeping", "cat", "dog") == "The dog is sleeping"
  assert replace_word("The sky is blue", "brown", "green") == "The sky is blue"
  assert replace_word("", "old", "new") == ""
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)