Before the sample data installation process, all original data of your website is automatically backed up into a backup file located in folder /backup inside the joomla_root_folder/templates/jsn_epic_XXX/, where XXX is the template edition on your server. Technically, this is the zip file which contains a plain text file with SQL instructions that need to be executed on your Joomla! database.
Below are the instructions on how to do that by using software phpMyAdmin 3.2.1.
After script execution, you can get back to your Joomla website to check how the original data was restored.