When you are becoming an uncle what habits should you change?

None, its not your kid. Live your life, just like you do now.

Just try and set a good example for the kids

i would just be a good example for your niece or nephew

One thing our kids' uncle had to change was how often he said words little kids shouldn't be hearing. He was admirable about it.

Based on your past questions one habit you should START is regular mental health care re-evaluation.  Your questions about addiction are particularly concerning.  

There is no official list of things you "should" change, because it's entirely subjective. And since we don't know what "bad" habits you currently have, we have no way of advising you about what you might want to change.