Download the Debian package to the server and install the package.
For 32-bit systems:
$ sudo dpkg -i vpnportal_1.1-201609061733_i386.deb $ sudo apt-get -f install
For 64-bit systems:
$ sudo dpkg -i vpnportal_1.1-201609231053_amd64.deb $ sudo apt-get -f install
Now you have to wait some time until the installing procedure is generating the secrets and configuration files.