Updating the theme can be made in 2 different ways. Both will lead to the same result, but only one is easier for some unexperienced clients.
Method 1: Using the official Envato Market Plugin. See tutorial.
Method 2: By downloading the theme from your ThemeForest Downloads section.
This method takes a bit longer and requires FTP or cPanel. First of all backup the theme. After making the backup you have to delete the theme from the server, upload the new theme archive and unzip it under the /themes/ directory. That’s all.