Howto Update Coreos with Update Engine

Get engine options:
update_engine_client -help

Get update status
update_engine_client -status

Force Coreos an immediate update check:
update_engine_client -update

If the update fails you can check the logs of the update engine by running:
journalctl -u update-engine -o cat

If you want to download another update you may need to clear the reboot pending status:

update_engine_client -reset_status