How do I move the site topic?

If you look at this site the issue is instantly obvious. the mobile version is very important for business.

I do not know how to get into and edit the style.css or any file using wordpress.

I’ve already altered some of the css. see: https://wordpress.org/support/topic/mobile-top-menu-cuts-off-my-head-in-banner-photo/#post-10578293 . i would like to keep this change and hope any further editing does not undo the above.

The page I need help with: [log in to see the link]