It's \renewcommand.
Don't use it lightly. It's usually much better to define your own macros as opposed to overriding macros whose definitions have deep meaning.
This looks like an "XY" problem. Underneath the hood you are facing problem X, something about which you haven't told us a single word. You somehow see that approach Y is a solution to your underlying problem, so you ask how to do Y. It's usually much better is to ask us about X directly.
So, what's your real problem, and what is motivating you to want to change \mathbf?