editing media.php

I edited a bunch of things in wp-includes/media.php – I know this stuff is going to get overwritten unless I move it somewhere else. What should I do to preserve the changes? It’s simple stuff in the outputted HTML that I had to change, but I don’t want it to get overwritten.

If the solution is to write a plugin, like I fear it might be, is there a good starting point for that?

Thank you,
Kim