Following the global variant, here is the AT&T variant of HTC One A9 guide to restore your device to stock firmware with the latest RUU file. The latest software upgrade to version 1.27.502.5 for the AT&T variant brings battery and camera improvements along with the latest Marshmallow firmware with the latest Sense 7 UI. The One A9 device comes with Android 6.0 Marshmallow firmware out of the box and is being updates for security patches and bug fixes regularly. The stock firmware from HTC comes in the form of an RUU file and the installation procedure is quite simple. The file for A9 weighs a lot, though, but this is a great deal when it comes to restoring your device to full stock.
Before you proceed, please confirm that this software upgrade to 1.27.502.5 is a newer version than what is currently loaded on your device. If it is the same version, there is no need to reload the software. Navigate to the Home screen, then tap All apps > Settings > About and locate the Software information. If your Software number is less than 1.27.502.5, you should proceed with the upgrade.
Moreover, confirm if any updates are available for your device from the AT&T’s software update section. However, if for any reason you have rooted or modified the firmware on your device, then you may not receive any updates over the air. Hence, to receive the OTA updates, you will need to revert back the changes and get stock firmware installed. To do so, the RUU.exe files is the easiest option available. There are many techniques you can you to install the stock Marshmallow firmware on your HTC one A9, provided you possess the correct file for that matter.
Hit the download section below to get the latest RUU file for your device. To know how to install the firmware, get to out installation tutorial guide from below. The guide involves 3 methods to flash the RUU file. The first one needs an RUU.exe file, which is suitable here, and the other 2 methods require a ZIP file.
During the upgrade process, the device will reboot multiple times and show software upgrade screens. Do not remove the AC charger or long-press the Power button as this might stop the update and your device may be rendered inoperable!
Download Marshmallow for AT&T HTC One A9
The links are now live. Also, visit HTC News Page for the latest version.
AT&T HTC One A9 Maintenance Release
→ Download | File: RUU_HIA_AERO_UL_M60_SENSE7GP_ATT_Cingular_US_1.27.502.5.exe
HTC One A9 RUU.zip file
→ Download | File: RUU.zip [Link not available]
How to install Stock Marshmallow Firmware on HTC One A9→
Install TWRP on HTC One A9→
Source: HTC News Page
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