Threre are two ways to 'get back to the local box'....
First you can put the astrobox IP address into your browser....It will ask you to log in and in a typical set you will just enter the same credentials that you use to get to the astroprint web site.
The other way is to go to the Astroprint website and click the 'Monitor' icon on the dashboard.
This will take you to the online monitoring app where you can choose your astrobox and click camera/controls button... this takes you to another screen where you can 'launch astrobox' with which will then take you to the local web page for your Astrobox. (on my system it opens in a separate tab and I just leave both the web and the local table open. This local tab is the same page you would get if you use the IP address method above. From there you can upload G-Code via the File Manager.
Given the number of clicks, I prefer just keeping a link in my browser to the local Astrobox address.
It helps if you set your astrobox up with a fixed IP on your LAN.