input: Write a python function to update volume percentage and assert if the function works as expected.
  assert update_volume(0, 0) == 0
  assert update_volume(100, 100) == 100
  assert update_volume(50, 75) == 75
  assert update_volume(30, 20) == 20
output: def update_volume(current_volume, new_volume):
      return new_volume