input: Write a python function to replace a substring within a string with a new substring.
  assert replace_substring('Python is very old', 'old', 'new') == 'Python is very new'
  assert replace_substring('hello world', 'world', 'universe') == 'hello universe'
  assert replace_substring('abcdabcd', 'abc', 'xyz') == 'xyzdxyd'
  assert replace_substring('aaaa', 'a', 'b') == 'bbbb'
  assert replace_substring('no match here', 'xyz', 'abc') == 'no match here'
output: def replace_substring(s, old, new):
      return s.replace(old, new)