If, while making cheese, you accidentally heat the milk too high there’s a quick fix – add a few ice cubes. Only add a couple cubes at a time, and keep checking the temp as you go. If you add them too quickly you might drop the temperature too much. Don’t worry about the extra water the ice cubes are adding – this small volume will simply be expelled with the whey. Only do this if the milk is too warm by a reasonable amount – this isn’t a good solution if, for example, you went 60F over your mark. For a large temperature discrepancy you’re better just letting it cool or using a cool water bath.