input: Write a function to replace a given string with another string in a sentence. The function should take the original sentence, the old word to be replaced, and the new word to replace it with. It should return the updated sentence after the replacement.
  assert replace_word("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_word("The sky is blue", "blue", "green")=="The sky is green"
  assert replace_word("Python is fun", "fun", "awesome")=="Python is awesome"
  assert replace_word("I love programming", "programming", "coding")=="I love coding"
  assert replace_word("", "walking", "swimming")==""
output: def replace_word(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)