Best way to import new design

Sorry if this has been answered but Im trying to find out the best way to import a new website I designed.

I recently upgraded my website.

I designed the new website with a different theme/pages/everything on a subdomain.

Now i want to replace everything on main domain with the content on the subdomain but keep the domain name. Im also concerned about blog content. I dont want the links to be changed in any way.

is this easier done in the ftp or through wordpress importer? other options?

Thanks