TrueMag has an auto-update feature which you can receive notifications about update. You will need to fill in your Envato API. See this: Update theme and plugins
However, some updates contain changes in plugins. With plugins, you will have to uninstall the old ones and re-install the latest versions which can be downloaded from ThemeForest. The list of changes can be found in the profile page of TrueMag.
To not affect user experience, you can use FTP or another file manager method to upload & override themes & plugins files.
Existing data will remain.