Creating and exporting Test Site

Hello,

I am about to buy and install a custom theme and will be making changes to it. My question is, How do I setup a test site on the same domain where I can make changes to the site without affection my production blog. Also once that’s set up, how do I export the test site with the changes and overwrite my production site without destroying blog posts already on the site.

I have done some HTML coding before but never have managed a site or made site changes.

Thank you for reading my question!