input: Write a python function to replace a substring within a string with another substring.
  assert replace_substring("hello world", "world", "there") == "hello there"
  assert replace_substring("hello world world", "world", "there") == "hello there there"
  assert replace_substring("abcdef", "cd", "12") == "ab12ef"
output: def replace_substring(s: str, old: str, new: str) -> str:
      return s.replace(old, new)