So long as the versions of Eshop you go from and to have the same database schema (same tables etc) then its as simple as doing a Mysql dump using your web hosting control panel (Cpanel, Plesk, etc) and then doing an import.
Though if all you are doing is making a new template or design, this should have no impact on your database at all, as all template changes are (usually) file-based and not database driven.
Best advice though is to login via FTP to your webhost, copy all the files from your website root into a new directory such as yourdomain/newsite then duplicate the database with a different name and setup the joomla config file on the /newsite to use the new database. Then do all your edits on /newsite and once complete, you should just be able to copy those files back to your root with no issues. This helps as it gives you a URL you can get to in a browser while you work on the new design, i.e
www.yourdomain.com/newsite
I am oversimplifying this a bit, but not much. as long as you make backups, and work on a duplicated site you make at /newsite you should be fine.
