If you are still on Android 6.0, you can update to the final official version (Android 7) using these methods:
: Installing these requires an unlocked bootloader and a custom recovery like TWRP . oppo f3 android 8 update download full
However, if you are determined to run Android 8 on this device, your only option is to install a . This requires unlocking the bootloader and installing a custom recovery. If you are still on Android 6