At the start of the learning curve this advice may not help a lot, but the conventional wisdom is: first create a unit test for the code you are about to change, then make the change. The idea is that you write a regression test that confirms the current behavior doesn't get broken by changes that you make. It's smart to also write a test the confirms the expected new behavior works too (when it's implemented of course).

The other important thing is to use a revision control system. That allows you to easily back out changes and to compare what you have now with some past version.