If It Ain't Broke, Don't Fix It
The most important software development principle that everyone should be following is “If it ain't broke, don't fix it.” I know, I know, it's not really a principle coming from the realm of computer science or software development, but it should be. Adhering to this sage life advice is just as beneficial when writing code, as it is in everyday life. Forget about trying to master the five SOLID design principles if you can't even follow this one.
If you're constantly trying to fix stuff that wasn't broken to begin with, you're in for a lot of headaches in your software development career. Not to mention, headaches for other developers in your team. If you find a piece of code that is working, that is not causing bugs, that is not causing performance issues, you best leave that piece of code alone.