Advice on how to update old website with new on the same domain

I currently have a domain of my website hosted on cPanel made with Wordpress, and i want change it to a new theme i have got, but also how can i start working on the theme of my new website without removing the existing website, because it would take me 3-4 hours a day to make changes to the new site them. Can i do a Addon domain and upload the theme there ? and later when done delete the old website domain when done and then is it possible to change the new domain url to the old one once deleted. If this above step is possible then how to i open it with wordpress admin to edit as the old site was made by a friend and i dont know much.