Assuming there is one (it's only updatable to 5.0), you'll get it when AT&T sends it to you. If you're not on AT&T, look in the LG section at XDA Forums to see if anyone has captured one (they normally delete themselves after they install the update). If you are, and the update was released, your local AT&T Device Support Center can flash it for you.
Are they both LTE versions from AT&T? Are both of them on the AT&T network? Carrier-specific devices need to be on that carrier's network in order to get the proper OTA system update.