Installed LineageOs16 on oneplus5 but I get black due to wrong firmware update how I can fix it?
You are close to fix the issue and have the lineageOS in your device. The only thing left to do in order to fix the issue is to boot into the recovery once again ( As far as I understand the twrp v3.3.1 is installed and temporaly booted into the oldest one).  

Please follow the following steps:


Boot into recovery mode via pressig Volume Down + Power
Then on twrp enable the sideload.
Run adb sideload lineage-16.0-20191220-nightly-cheeseburger-signed.zip (lineage-16.0-20191220-nightly-cheeseburger-signed.zip is the latest one for now please replace it with the build of lineageos you desire as long as it is the version 16 of it)


So in other words you just need to reflash the lineageos using the latest twrp version you installed.