Edit /etc/apt/sources.list and add the following lines:
Unstable edition
# qgis unstable
deb http://qgis.org/ubuntugis-nightly xenial main
deb-src http://qgis.org/ubuntugis-nightly xenial main
deb http://ppa.launchpad.net/ubuntugis/ubuntugis-unstable/ubuntu xenial main
N.B. change xenial to the codename of whatever release of Ubuntu you have installed.
Stable or LTR editions
Change these lines in accordance with the instructions given on the Qgis download page.
Then enter these commands (all as root):
apt add-repository ppa:ubuntugis/ubuntugis-unstable
apt update
You will get this error message
W: GPG error: http://qgis.org trusty InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 3FF5FFCAD71472C4
To fix this you need to run the following command:
apt-key adv –keyserver keyserver.ubuntu.com –recv-keys 3FF5FFCAD71472C4