After purchasing the theme, please go to Downloads area on your ThemeForest account.
Click on Download button, and select Installable WordPress file only.
There are two download options available, “Installable WordPress file only” – you get only the WordPress installer zip file, and “All files & documentation” where you get the installer zip file, the documentation and other files included in the full download package.
On your WordPress Dashboard, please go to Appearance – Themes and click on Add New button.
On the next screen, please click on Upload Theme button
Click on Choose File button and select the theme installer zip file that you got from ThemeForest.
Click on Install Now then click on Activate button.