To update latest version theme on themeforest.net

Step 1: Download theme

Go to  themeforest.net and download your theme.

Step 2: Backup data

Backup your site( data + file theme), you can use https://wordpress.org/plugins/duplicator/ plugin to buide a package data

Step 3: Update

#1. If you use child theme (https://codex.wordpress.org/Child_Themes)

- Remove or rename old version theme on your site

- Install new version theme: upload file .zip via Add new theme and install it.

#2. If you don't use child theme and don't edit theme

- Remove or rename old version theme on your site

- Install new version theme: upload file .zip via Add new theme and install it.

- Active new version theme

#3. If you don't use child theme and edit theme

Case 1: Compare

- Compare with old version theme( use tool compare https://www.scootersoftware.com/ )

Case 2: Use child theme

- Edit old version theme to child theme: (https://codex.wordpress.org/Child_Themes)

- Remove all files that you not edit on old version theme

- Install new version theme