A full system backup is critical as this process wipes all data.
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
In TWRP, select Wipe and perform a factory reset. For a "clean flash," go to Advanced Wipe and select System, Data, Cache, and Dalvik. Flash the Firmware: Tap Install and select the Android 7.1.2 ROM zip file. Swipe to confirm the installation. A full system backup is critical as this
After the ROM is installed, repeat the installation process for the GApps zip to ensure you have the Play Store and other Google services. For a "clean flash," go to Advanced Wipe
The phone's bootloader must be unlocked to allow custom software.
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