You need to add the following entry in /etc/apt/sources.list
or a new file in /etc/apt/sources.list.d/
:
deb http://deb.torproject.org/torproject.org xenial main
deb-src http://deb.torproject.org/torproject.org xenial main
Then add the gpg key used to sign the packages by running the following commands at your command prompt:
gpg --keyserver keys.gnupg.net --recv A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89
gpg --export A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89 | sudo apt-key add -
You can install it with the following commands:
sudo apt update
sudo apt install tor deb.torproject.org-keyring