If DJI GO is not prompting you to install the update, then connect your remote controller to your computer via a micro USB cable and update it with the DJI Assistant 2 application.
The DJI Assistant 2 application will not prompt you to update. It'll show you the current version of the firmware that's installed on the remote controller and allow you to upgrade it if a newer version is available.
I found that the easiest way to update firmware is on the dji 4 app. Click the status bar which is saying no image, no connection etc. the first line should say normal. There click the arrow and should prompt you to update rc.