I purchased the Theme Forest, Presso theme and have version 2.1.0.
To update my theme to the latest version, do I have to buy it?
no, you don’t need to buy a new license for getting the latest update.
If you purchased the theme from Envato market then to get auto-update features you can use Envato Market WordPress Plugin. 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:
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.
Thanks