This update effectively gives your phone a new lease on life as the changes in Windows 10 Mobile are pretty huge. In a post on AT&T’s forum, “The upgrade to Windows 10 is an ‘opt-in’ experience. You will need to download and install the Windows Upgrade Advisor app on your device and enable it for the upgrade. Enable must be checked to get the upgrade. The package will be pushed via OTA directly to the device.”
The carrier also points out that users can choose to postpone the update, like maybe if they don’t have storage, or if they’re not connected to WiFi. “You will have the option to ‘postpone’ or ‘install’ the update immediately to your device. The software can be downloaded via a Wi-Fi connection. If you select to ‘postpone’ the installation of the update, the notification will pop up again in about two days. The notification cannot be postponed permanently.”
Given that the update is said to weigh about 1GB in size (AT&T claims it’s around 300+MB, but other reports are claiming otherwise), we guess downloading it over WiFi is probably a better idea.