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.