[TOS'd]

Try going to About Phone > System Updates > Check for Updates. That was how I got my updates on my OPO.

And see if it brings up anything. This route will might give you Android 6.0.1 and Cyanogen 13.1. Android 6.0.1 being the OS, and Cyanogen 13.1 being kind of like the "skin" applied over the OS.

Lineage did replace Cyanogen, so if you want to go that route, check out xda forums for instructions or possible the oneplus forums as well.