Apply new html template to existing wordpress blog

The design of my website has been updated and I need the templates of my wordpress blog to be ammended to match the latest design of my website. At the same time I would like the wordpress blog to be upgraded to the latest version. Because the blog is important to my business, I first need the blog (existing database plus templates) to be copied onto a test server, where the templates can be updated and tested. Once the new templates have been applied and I have tested that they work correctly, the templates need to be copied back onto the main server I will provide access to the servers, and a copy of the html template to be used. I will also do the testing as necessary. The person suitable to do this should be very familair with wordpress blogs, applying html templates to wordpress blog, php. The background to the project is that I created a wordpress blog 5 years ago and I customised the templates at the time myself but I really don't have the time now to remember how to make it all work so I really need an expert to make life simple for me. From memory the templates worked as a header, footer, and sidebar.