Protect yourself: put everything in writing, and decline to make any change that you are not confident that you understand. It is not your responsibility to fix

Protect yourself: put everything in writing, and decline to make any change that you are not confident that you understand. It is not your responsibility to fix this situation, and don't let yourself think so. If you change the code, you become implicitly part-responsible for it. And if you change complex code that does not have tests, you will break something.