Update Oneplus 2 with Oxygen OS 2.1

Step 1. Download the full ROM zip file of Oxygen OS 2.1 here and don’t unzip or extract the folder.
Step 2. Download the SuperSU zip file here to get the root access on your phone. Again don’t unzip or extract the folder.
Step 3. Once both the files are saved on your PC move both the downloaded files on your Phone.
Step 4. Boot your phone in TWRP recovery mode to flash these files. See here how to boot in recovery mode.
Step 5. Once you on the recovery mode. Select the Wipe option > Advanced Wipe > Select system, cache, Dalvik and Wipe them.

Step 6. GO back to the main menu, Select Install And Flash the Oxygen OS Zip file.
Step 7. Now Flash the Super SU zip file to get the root access on your phone.
Step 8. Go back and Reboot the device to the OS.