1. Get notified
To make sure you are always up to date make sure you have subscribed to be notified.
Go to your themeforest.net account -> downloads
2. Use child theme
If you plan to modify your theme, we encourage you to use Child theme, simply when installing our theme, upload one more theme which is called apetita-child.zip and choose child theme under Appearacne -> Themes
This way if you update the main theme you will not lose any of your customization.
Read more about benefits of using child theme here https://codex.wordpress.org/Child_Themes
3. The update itself
Download the latest version from themeforest.net, log in to your server with FTP client i.e. FilleZilla and replace theme folder with the new one.
If you are not sure what you are doing, please ask for assistance and always backup, just in case, and most important please use Child Theme.