[arkh]

Get a copy of "Working effectively with Legacy Code".

The definition of Legacy Code for the author is "untested code". The book is mainly a list of situations a code base can be in and how to add tests.

Once your application is end-to-end tested you can play with the code with better peace of mind. Just adding those tests require you to get all the specifications the app has to fulfill so it is a good way to learn them.