Sites like HardReset.info often maintain databases of official stock firmware for older devices. Important Precautions

No. The local upgrade via Settings preserves your apps and data. However, downgrading or using the recovery method with a full package may require a wipe.

Upgrading from Android 6.0 Marshmallow to 7.1.1 Nougat brings several functional upgrades:

To check for the most stable official version provided by OPPO: On-Device Check Software Update to see if a version is waiting for download. System Upgrade Tool : You can use the official OPPO System Upgrade Tool