At first thing always should to purchase item from the client (website owner) own envato account. So that they will be able to update the theme regularly.
Secondly if your client purchase the theme again and the theme has updated version then they or you as developer can update the theme.
You can use Envato Market WordPress Plugin to get auto update features. This plugin helps customers receive updates to their premium wordpress Themes & Plugins purchased through Envato Market (ThemeForest & CodeCanyon).
you can download the plugin from here:
Using this Envato Market WordPress Plugin you will see:
- Latest version (need to update version) number
- Details view option for the latest version
otherwise you can download the update version from Envato Market Downloads page and update manually using ftp.
Note: Better to keep a full backup before doing the update.