Sign into the Gateway's User Interface using 192.168.0.1 in the address bar of your internet's browser.
From there, you need to click the Edit button next to Broadband Connection (Cellular). It'll usually be the last one on the list, but check first before clicking.
From there, scroll to the bottom and click Settings.
After scrolling down slightly, you will finally be able to view and update the APN in the modem.