I’m looking for a way to center align the Header Menu

Link to site? I could tell what would work for the Twenty Twelve theme but the CSS selectors might not be the same.

What @markrh said, however if you’re using a custom theme it may provide some options in the customization options. You can get to those by going to the admin page, then clicking on Appearance->Customize

Edit: I noticed you added the tag “REHUB CHILD THEME” – if there’s no option in the theme, you can try to contact wpsoul (the creators of rehub) and see if they can help you.