Désolé, je ne parle pas français, mais on m’a demandé d’aider, alors soyez indulgent. (Google Translate, désolé encore 
)
Make a backup of your personal data, you know … pictures, contacts, ringtones etc., they will be erased!
For easy success, just follow the excellent 5-step guide at LineageOS 14.1 with OTA updates for FP2, section “When coming from any other OS:” (That’s important!).
I don’t know what else to say. If something goes wrong along the way, boot into TWRP again, wipe the whole phone (but not the Micro SD card), start again, read the guide again, you probably forgot something 
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