If I want change the styling of
<a class="brand" href="#">Product</a>
How I should to do?
I think the choice is between editing .brand in bootstrap.css or create new style.css and replace .brand in new css file for the better.
You should definitely create a new custom stylsheet and override the classes you want to modify. That way you can always go back to the default styling provided by bootstrap, if you change your mind.
You then have a solid base of CSS, to which you can always go back. If you want to redesign your page, you can just exchange your custom stylesheet with a new one. Instead of getting a completely new bootstrap file to be able to start over. Also, if you just want to go back to the default styling on just a few elements, it would be a mess to remember what changes you have made.