Activate the auto update simply by going to
WP Dashboard > Envato Toolkit >
Settings and enter your Envato Username in the field "Marketplace Username", Your Secret API Key to field "Secret API Key". Select whether you need or don't to save backups by putting check mark at the "Skip Theme Backup" and click
Save Settings. The theme will be automatically updated when the new version is available.
Info! How to find Secret API Key:
You may find the information about API Key generation in the Help Section on Themeforest.