input: from typing import List
  
  
  def replace_string(sentence: str, old_word: str, new_word: str) -> str:
      """ Replace a given string with another string in the input sentence.
      >>> replace_string("I love walking on the beach", "walking", "swimming")
      "I love swimming on the beach"
      >>> replace_string("Hello world", "world", "Python")
      "Hello Python"
      """
output:     return sentence.replace(old_word, new_word)