As with previous OTA updates to Windows 10 Mobile, the download for the AT&T Lumia 830 will require the use of the Upgrade Advisor app from the Windows Store. Here's what's AT&T says Lumia 830 owners should do:
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. 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.
AT&T adds that the download size for the update will be 333MB Update Looks like that AT&T got that wrong; its more like 1GB in size.
Once it is installed, the OS version number will be 10.0.10586.218 and the firmware number will be 02179.00000.16201.54051.
Thanks to Long Syntax for the tip!