Step 2. Download Android 6.0 Marshmallow CM13 zip file here and save the file on your phone’s internal memory
Step 3. Now Download Android Marshmallow compatible Google Apps here , You need this zip file to install all the Google application on your Oneplus one.
Step 4. Once both the Zip files are saved on your phone. Boot your Oneplus into TWRP Recovery Mode. To do this Turn off your phone and Turn on the phone while holding Power and Volume Down buttons.
Step 5. From the TWRP recovery main menu, Go to Wipe and select Swipe to Factory Reset Option, This will wipe cache and dalvik cache form your phone.
Step 6. Now go back to main menu of recovery and go to Install and find the zip files you saved on your OnePlus one Internal memory.
Step 7. From here, Select the CM 13 ROM.zip file that you downloaded on step 2, after that select Add more zips option and select GApps.zip > Swipe to flash.
Step 8. Wait for the flash to finish and press Wipe Cache & Dalvik.
Step 10. Press Back and press Reboot System.
First boot will take some time, wait for 5 minutes to apply the CM 13 update on your phone. Now your phone is running on the official CM 13 Android 6.0 Marshmallow. Enjoy!
Author of OnePlus Developers Blogger Site
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