First add the following line to /etc/apt/sources.list

deb http://www.ubnt.com/downloads/unifi/distros/deb/debian debian ubiquiti

Next add the GPG key for UniFi and mongo-10

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv C0A52C50

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv 7F0CEB10

Update the system

sudo apt-get update

Install the UniFi software

sudo apt-get install unifi-rapid

Once it is finished installing you should be able to browse to https://yourserverip:8443 to access the UniFi web page.