Hi,
1. Try deactivate the theme by going to Theme Options.
2. Delete the theme by login to FTP using filezilla or any other FTP file manager and navigate to wp-content/themes/xstore and delete the theme folder.
3. Login to themeforest and download the latest version of xstore,
4. Install it via FTP or WordPress,
5. Activate the theme by the license key.
Optionally if you want to delete all the content in the site, you can use this plugin https://wordpress.org/plugins/wordpress-database-reset/ ( use with caution, it deletes everything )
Let us know,
Thanks,
laranz.