Bootstrap Minified

There are several bootstrap css that I found in bootstrap.min.css that I need to change.
I already have a theme child but I was not sure what would be the best way. I tried to add a css part in the styles.css of the child theme but it didn't pick up. In my situation: theme/css/bootstrap/bootstrap.min.css
How to convert the css to min file right away? Your assistance will be greatly appreciated.

Thanks in advance