Saving changes to CSS

I need the top navigation bar to wrap. I read up on how to do that; to change it from nowrap to normal in the CSS when I hit F12. However, it will wrap as long as I am on that screen, but when I move to another screen it just goes back to "nowrap". How do I make it stay? is that a "save" button somewhere, because I can't find one.....Please assist.

Thank You so much for your help

Lost and needing directions!!!