Dell Edge Gateway 5000 Core 16 root user

I bought Edge Gateway 5000 on eBay, the system has been reinstalled with Ubuntu 15 Desktop.

I reinstalled with Ubuntu core 16, and follow Edge Gateway 5000 manual for testing, but some command ask for root user: 'su -', this is what I don't know how to enable root login on Ubuntu core 16.

Also, I can't find pre-build Ubuntu core 16 for Dell Edge Gateway 5000, only Core 15 version, where I can download the pre-build version?