What ONE thing? There were MANY one things that changed my game for the better. If you are really paying attention, you should find that every little thing you change, forces you to change something else you were doing as a band-aid to cover the previous problem.
While there are many things that can change your game for the better, I suspect that someone asking for just one thing is looking for a magic pill. None exist. What one person does to change his game for the better, isn't necessarily something someone else should do. It's not just changing something, it's knowing why it needs changing, and what needs to be done to change it correctly.
Just an example- an elbow drop might be a very bad thing for person A. But, for person B, it is not a problem. Just because person A changed that for the better, doesn't mean that it will help person B. One needs to change things that are a problem for them, not what is a problem for others.