input: Write a function to replace a given string with another string in a sentence. The function should take three parameters: the sentence, the old word, and the new word. It should return the updated sentence.
  
  assert replace_string("I love walking on the beach", "walking", "swimming") == "I love swimming on the beach"
  assert replace_string("The quick brown fox", "quick", "lazy") == "The lazy brown fox"
  assert replace_string("Python is fun", "fun", "awesome") == "Python is awesome"
  assert replace_string("Learning programming", "programming", "coding") == "Learning coding"
  assert replace_string("Hello world", "world", "universe") == "Hello universe"
output: def replace_string(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)