A. Use auto updater functionality
For 99% of the users, there will be the auto updater functionality working out of the box. You simply go to WP Admin -> Dashboard -> Updates, and here you click the update, thats it
B. Update the theme manually
If you dont see option to update the theme, it could be for many reasons. For example, you renamed the theme files and the updater cannot identify the theme anymore. Dont worry, even when updating automatically, you dont loose your theme settings. You have to do this:
1.) Download latest theme package from themeforest
2.) Go to WP Admin -> Appearance -> Themes and <strong>delete</strong> (yes, delete), the current theme
3.) Upload the new theme package from WP Admin -> Appearance -> Themes
After that, your theme should be updated and you should be able to run the latest version.