Cyanogen has started rolling out Cyanogen OS 12.1 update on OnePlus One, which brings Android 5.1.1 Lollipop along with a host of new features, security patches and bug fixes to the phone. Since it takes little longer time to reach the OTA (over-the-air) update on all OnePlus One, tech-savvy users can manually install the new firmware on their OnePlus One.
The below guide lets you know how to flash the Cyanogen OS 12.1 update manually on OnePlus One (YOG4PAS1N0) manually. Users who are running on the latest Cyanogen OS 12 (Android 5.0) needs to download a small portion of the update (304MB), other users need to download the full ROM (645MB).
Cyanogen OS 12.1 (YOG4PAS1N0) update for OnePlus One patches Android’s Stagefright security hole. It will also fix bugs that were found in the previous build. In addition, you can experience features like Smarter Screen, CM Browser, redesigned app drawer, enhanced AudioFX and more on your phone.
How to Flash Cyanogen OS 12.1 (YOG4PAS1N0) Update Manually on OnePlus One
Prerequisites: Before flashing the ROM, your OnePlus One should be rooted and custom TWRP recovery installed. Also, install proper driver on your computer.
Important: Backup every data on your phone before flashing the ROM
WARNING: This procedure will void your device’s warranty. Follow the instruction at your own risk. We will not be liable if any damage occurs to your device and/or its components. It is recommended for technically talented users only.
1. Download Cyanogen OS 12.1 (YOG4PAS1N0) zip file (cm-12.1-YOG4PAS1N0-bacon-signed.zip) [HERE] and put it on your OnePlus One.
2. Now, boot your OnePlus One into TWRP recovery. To do so, disconnect the phone from the PC (if connected), then power off it, and then press and hold Power and Volume Up button together for few seconds.
3. From TWRP recovery, select Wipe and clear the phone memory.
4. Now, select ‘Install’, choose the ROM zip file (you have copied on your phone) and swipe to confirm flash.
5. Once the flashing process is finished, reboot your phone by selecting ‘Reboot system’ option from recovery menu.
It will take longer time to complete first boot. Once booted up, check your device for the new Cyanogen OS 12.1 (YOG4PAS1N0) update by going to Settings -> About phone. (source)