Go to Envato Market plugin and put token in order to get automatic update.
Once you generate token put that in the token text box and save. After that you can find the notification on composer theme ( if any update available ) in Appearance -> Themes.
1. Go to Envato Market plugin and click Generate a Personal Token.
2. New Page will open, choose the options like in below image. (You need to login if required, use the account details you brought the theme)
3. click create token. you will see this.
4. Copy the token and paste it in token field and click save
5. Once saved go to Appearance > Theme. You will see update in top of the composer theme preview image. Click it to update.
Please note that after every the theme update you also need to update all the recommended plugins including composer Core plugin, Visual Composer, Ultimate VC Ad-dons, Slider Revolution, etc in Appearance -> Install Plugins -> and update plugin if available.