input: Write a function to replace a substring in a string with another substring.
  assert replace_substring("Hello world!", "world", "universe") == "Hello universe!"
  assert replace_substring("Hello there!", "there", "world") == "Hello world!"
  assert replace_substring("apple banana apple", "apple", "orange") == "orange banana orange"
output: def replace_substring(s, old, new):
      return s.replace(old, new)