1.Download Android 6.0 AOSP ROM .
2.Download TWRP Recovery 2.8.6 .
See here how to install TWRP recovery
3.Charge your phone +80℅
4.Enable USB debugging.
5.Ensure that the device is rooted with CWM or TWRP recovery installed and its bootloader is unlocked.
6.Connect and mount the phone's USB mass storage on computer with USB cable.
7.Now, copy the downloaded AOSP ROM zip file to the root folder on the phone's SD card.
8.Power off the phone and disconnect its USB cable from the computer.
9.Boot the device into Recovery Mode by pressing and holding Volume Down and Power buttons together.
10.Once the phone enters TWRP Recovery, perform a full data wipe task by clicking Wipe button. Then swipe on the Swipe to Factory Reset option at the bottom.
11.Then return to the main recovery menu and tap on the Install button. Choose the zip file that you coped earlier to your phone's SD card from the list.
12.Confirm the installation on next screen by swiping on the 'Swipe to Confirm Flash' option and the firmware installation should begin.
13.Once the ROM installation completes, tap on Wipe Cache/Dalvik button to prevent boot loop issues.
Finally, tap on Reboot System button in the Recovery menu. The phone will now reboot and the first boot may take about five minutes to complete. So, please be patient. Your OnePlus One is now running the latest Android 6.0 Marshmallow via AOSP custom ROM. Go to Settings > About phone to verify the firmware version installed.
[Source: XDA,ipt]
