UniFi Controller APT Steps

1) Add new source:
echo 'deb http://www.ubnt.com/downloads/unifi/debian stable ubiquiti' | sudo tee /etc/apt/sources.list.d/100-ubnt-unifi.list

2) Add GPG keys (must have sudo access and wget installed):
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 0C49F3730359A14518585931BC711F9BA15703C6
echo "deb http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.4.list
sudo apt update

3) Update:
sudo apt update

4) Install and update Unifi controller
sudo apt install unifi

Source of my information: https://help.ubnt.com/hc/en-us/articles/220066768-UniFi-How-to-Install-Update-via-APT-on-Debian-or-Ubuntu

Major overhaul in the looks of version 5.9.29.x!