Earlier today, the Cyanogen OS 12.1/Android 5.1.1 update for OnePlus One started rolling out. The update brings with it a handful of new features and performance improvements and should also fix the battery life issues that most One owners have been facing after upgrading their handset to CM12S.
You can find a list containing all the changes introduced by Cyanogen OS 12.1 here.
Since the OTA update is being rolled out in stages, it will take up to a few weeks to be available for all OnePlus One owners out there. Thankfully, since Cyanogen has also provided the direct download link of the OTA update, impatient OnePlus One users can sideload the update manually on their device by following the steps below.
P.S.: Following the steps below will not touch the data stored on your phone in anyway. However, it is still recommended to create a backup just in case things go wrong.
Step 1: Download the incremental Cyanogen OS 12.1 update for the OnePlus One from here and transfer it to the internal storage of the device.
Step 2: On your OnePlus One, enable ‘Developer Options’ by going to Settings -> About Phone and tapping on ‘Build Number’ seven times. Now, head back to Settings -> Developer Options and enable the ‘Advanced Reboot’ menu.
Step 3: Long press the Power button to bring the Power menu and then select the Reboot option. From the pop up that appears on the screen, select the ‘Recovery’ option.
Step 4: Once your device boots into recovery, select the ‘apply update’ option followed by ‘apply from internal storage.’ Then navigate to the OTA update file that you had transferred to your One in the first step and confirm your selection by pressing the Power button.
Once the ZIP file has been successfully installed, reboot your device from the Recovery menu to enjoy Cyanogen OS 12.1 on your One.
The above steps will also work for you if a custom recovery like TWRP is installed on your OnePlus One. However, your device must be running on Cyanogen OS 12 itself and not any other custom ROM. In case your system partition is modified in anyway, it is more than likely that you will get a Status 7 Error during the installation process.
If the above ZIP does not work, you will have to manually flash the system partition from the OnePlus One factory image and then repeat the steps listed above to enjoy Cyanogen OS 12.1 on your handset.