input: Write a function to replace all occurrences of a substring in a string with a new substring.
  assert replace_substring("Hello World, Hello", "Hello", "Hi") == "Hi World, Hi"
  assert replace_substring("abcabcabc", "abc", "xyz") == "xyzxyzxyz"
  assert replace_substring("aaa", "a", "b") == "bbb"
output: def replace_substring(s, old, new):
      return s.replace(old, new)