The Oreo update is rolling out to Nokia 5 in Tunisia, India and Indonesia, while the Nokia 6 is only getting it in India. In the next few days, more markets are expected to see the new software.
The update can be downloaded on Wi-Fi for now but will be available later for download on mobile data. The Nokia 5 file update size is 866.7MB, while the original Nokia 6 file size is 933.8MB.
To update, go to your Settings>>About Phone>>System Update and upgrade manually. After successful upgrade, you will be on March 2018 security patch level.