After you might have purchased a theme at Themeforest, a zip package will be available for you to download via the "downloads" page. At the "downloads" page you should see a list of all the items you’ve bought. You can acces the page by hovering over your username at the top of the Marketplace page, when you sign in to your account.
The actual theme zip you will upload to your Wordpress site is inside the full zip package that you will download from Themeforest. Hence, you will first have to unzip the full zip package for you to see the actual theme zip. The full zip package downloaded from themforest will likely contain other files like documentation, licensing file, quickstart or demo file, help file etc
Upload Themeforest theme to Wordpress
To install the actual theme zip extracted from the zip package downloaded from themeforest, sign in to your wordpress site with your admin credentials and go to Appearance > Theme.
Click "Upload", browse to choose the zip file and click "Install".
If you upload the correct zip file, the theme will install susccessfully.
Below is a visual guide provided by Themeforest support team to guide you in the installation process.
Also, don’t forget to read the documentation in the zip package you download from ThemeForest. It usually explains how to make use of the theme. It may also contain some video tutorials.
Go to Themeforest Wordpress section here to sign up and download beautiful themes.