You have two available ways to install this theme: FTP Upload or WordPress Upload.
The .zip file you downloaded from Theme Forest contains a folder named pirouette.
Upload this folder on your server in your WordPress directory to /wp-content/themes/
The second way to install the theme is by logging in from your website /wp-admin to the WordPress Dashboard. From there, you should go to Appearance > Themes > Install Themes > Upload and select the file pirouette.zip from your computer. After clicking the Install Now button, the installation process is finished.