You have two choices:
- You can update the theme manually (download the theme package when an update is released and when uploading the theme zip file via WordPress backend, you will be asked if you want to overwrite the theme files with the new version)
- Using the Envato Market WordPress Plugin. Inside that link you'll find a guide about how to use the plugin and how to update the theme.
Do not delete any of the theme required plugins or you may loose all associated data.