The process is nicely described on Unifi website but for common good I am going to minimize it just to a set of commands you can run on Linux distros like Debian or Ubuntu:

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

2. sudo wget -O /etc/apt/trusted.gpg.d/unifi-repo.gpg https://dl.ubnt.com/unifi/unifi-repo.gpg or you can also do: sudo apt-key adv --keyserver keyserver.ubuntu.com --recv 06E85760C0A52C50

3. sudo apt-get update

4. sudo apt-get install unifi

5. in my case I also had to add: sudo apt-get install mongodb

6. Controller should be available on your public or private IP (try 8080 or 8443)