A full system backup is critical as this process wipes all data.
You must have TWRP (Team Win Recovery Project) or CWM installed to flash the ROM files.
You will need the LineageOS 14.1 ROM zip and a compatible GApps (Google Apps) zip for Android 7.1. Step-by-Step Installation Guide Oppo F1s Firmware Download 7-1-2
To install Android 7.1.2 on your Oppo F1s, follow these steps:
After the ROM is installed, repeat the installation process for the GApps zip to ensure you have the Play Store and other Google services. A full system backup is critical as this
The Oppo F1s originally ran Android 5.1 (Lollipop) and officially received an upgrade to Android 6.0 (Marshmallow) with ColorOS 3. OPPO India Community Update software – OPPO F1s - Android 5.1 Guide
Before attempting to flash a custom Android 7.1.2 firmware, ensure you have the following: Step-by-Step Installation Guide To install Android 7
At least 60% charge to prevent the device from shutting down during the flash.