input: Write a function to replace a given string with another string. If the given string is not found in the sentence, return the original sentence. The function should have three parameters: the sentence, the old word to be replaced, and the new word to replace with. Return the new sentence after replacement.
  
  assert replace_string("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_string("The sun is shining", "shining", "glowing")=="The sun is glowing"
  assert replace_string("This is a test", "test", "example")=="This is a example"
  assert replace_string("No change needed", "change", "replace")=="No replace needed"
  assert replace_string("Unchanged sentence", "missing", "word")=="Unchanged sentence"
output: def replace_string(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)