Assuming that you are just making theme changes, all you need to is upload our amended theme's folder to wp-content/themes on the live site. Job done.
With regard to your earlier questions:
Do I just need to register with WP, create webpages on the WP site and then publish them to make them live
I would certainly suggest that approach, yes. Otherwise you're back to physically restoring database backups etc.
do I need to ask/pay WP to host before the pages are live?
wordpress.org does not host any domains/sites apart from its own. Nor do you have to pay wordpress.org for anything related to WordPress. It's all free. But you do, obviously, need to have some paid hosting space and a domain name in order to run a live site. That can be sorted out by you separately.