Nexus 10 - Android 5.0.1 OTA upgrade, “no command” and suggested steps failed
If you are planning on installing a custom rom you need to install a recovery to flash the the ROM. TWRP is good

To get Marshmallow on the Nexus 10 Omnirom is your best bet. You'll need to flash gapps and supersu for root too.

The nexus 10 received 5.1.1 security updates until April 2016 that you can get OTA on a stock device or install manually, but manual installs will wipe the content of the device each time.

The Unified Android Toolkit is good but the information is out there on the Nexus 10 Cyanogenmod page on how to flash a custom ROM from the command line.