It depends on what kind of code you're talking about. If you simply want to change the theme or alter the HTML, there's a template system in PhpBB, but how you get to it depends on what version you're using.
If you want to alter the PHP, like I was talking about, you have to edit the files that make up PhpBB. You would, of course, have to know enough about PHP to get around and change what you need to be changed. There is no option in PHpBB to do this, as you are altering the files that actually make everything you see.