Please note three important things:
- Flash the Lineage recovery: TWRP is not supported right now.
- You need to install new gapps at the same time as you install the new official 18.1 build!
- You need to wipe your data: meaning that you will loose your previous data on the phone. Unfortunately this is necessary as we cannot decrypt the phone or use the lock passphrase in 18.1 anymore. Google has deprecated the old keystore mechanisms in AOSP and lineage does not have a hack for this. So please backup photos and other stuff.