To config a ThemeForest WordPress theme automatically update, please follow the guide:
What is ThemeForest WordPress Theme Automatically Update
Normally, you need to upload and re-install the zipped file to update a new version WordPress theme(from ThemeForest) when a new version is released. If you have configured AutoUpdate(this job for only one time), there would be a tip if the new version comes. Yon only need click once to update it.

Install & activate Envato(ThemeForest) Market plugin
For SEA, Arnold, Art WordPress themes, it is possible to install Envato Market plugin in WordPress wp-admin panel. Please click Appearance > Install Plugins, install and activate Envato Market plugin.

If you do not find the plugin in above list, please download the plugin here, install(upload) it manually(Plugins>Add New).
Enter the Token
Please find the Envato Market menu item at the left bar, click it to go to the plugin configuration page. You need to go to the Envato website to get the Token, click “clicking this link”(or click here) marked with 2 in the screenshot below.

It will navigate Envato API website, you would login with your Envato(ThemeForest) account. You will see the webpage(screenshot below), you could get the Token code on this page. Please enter a name, pickup the 3 options, click Create Token.

It will display the Token code. Copy it.

Paste the Token code in the Envato Market plugin page.

Check Updating
Click Themes tab, the items what purchased from ThemeForest will display. If there is the new update, the notice bar will display.
Click Update Available…

It will automatically download and install the new version theme. The green bar will display if it is completed.

BM Content Builder Update
For SEA, Arnold, Art theme, please update BM Content Builder plugin. It is required.
Go to Plugins > All Plugins, find BM Content Builder, click “Update Required”, it will guide you to finish the update.

If there is not the “Update Required” link, it isn’t necessary to update the plugin.