first of all to avoid any difficulties, please, do the backup of theme files and data base before any update.
After that please follow the steps below to update theme through FTP:
Download the zipped package from ThemeForest and extract to your desktop.
In the extracted package you will find archive Royal_v1.8.zip, which is the WordPress theme.
Extract zip file and upload only extracted royal folder to /wp-content/themes/ folder on your server with overwriting.
Please, re-save your Royal options after upgrading the theme. Navigate to admin panel > Theme Options and hit Save changes.