How to override a particular style rule

What is the correct way to customize a theme by overriding a particular rule?

I have edited style.css to override the rule .singular .entry-content {} to change the width to auto.

Is that the best way to do it? If I update the wordpress version, my copy of style.css will be over-written and I will lose my change.

I have edited style.css in the past for other reasons and keeping track of my edits can be troublesome.