Extract the zipped package downloaded from ThemeForest to your desktop, in the extracted package you will find the herald.zip file which is the installable WordPress theme.
You can install the theme in two different ways:
1. WordPress theme uploader
Navigate to Appearance -> Themes page to access the theme uploader screen and use the herald.zip file.
Watch the process in this short video:
Extract the herald.zip file and upload the extracted herald folder to /wp-content/themes/ folder on your server. You will have to use some FTP software, like FileZilla.
After uploading the theme, you have to activate it. Navigate to Appearance -> Themes page to activate the theme.