You can auto-update directly from the Themes panel in your WordPress Dashboard. To enable the auto-update:
Step 1: Login to your Theme Forest account and go to Settings > API Keys
Step 2: Enter a label name (ex: theme update) and click on the Generate API Key button.
Step 3: Go to Appearance > Customize > General > Automatic Theme Update
Step 4: Type your username and key in the Theme Forest Username and Theme Forest API Key fields.
Before updating is important to know that theme settings and content are not lost during the update process. You will lose changes to the theme files if you changed any files. In order to prevent losing the file changes we strongly recommend that you use child themes. Also we recommend that you look in the release notes before updating.
Go to 11. Child Themes to learn more about child themes.