Log in as root, download the package and run the dpkg tool in install mode:
wget <PACKAGE_URL>
sudo dpkg -i <FILE_NAME>.deb
For example, to enable the Ubuntu 16.04 Xenial repository:
wget https://apt.puppetlabs.com/puppet6-release-xenial.deb
sudo dpkg -i puppet6-release-xenial.deb
Update the apt package lists:
sudo apt-get update