Your link point to a .deb file which can be installed by dpkg, apt or the graphical package installer.

Just download the file and double click to install it. If that doesn't work, open the terminal, cd to your download folder cd ~/Downloads and run sudo apt install ./unifi_sysvinit_all.deb . You will then be asked to type your password blind. (It will not show how many characters you've typed, just hit enter when you're done). Wait for the installation to finish then you should find the app in the launch menu.