QGIS Server Build

First install some tools you will need for this instructions:

sudo apt-get install gnupg software-properties-common -y
Now install the QGIS Signing Key, so QGIS software from the QGIS repo will be trusted and installed:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 51F523511C7028C3
Add the QGIS repo for the latest stable QGIS (3.14.x Pi).

Note: “lsb_release -c -s” in those lines will return your distro name:

sudo add-apt-repository "deb     https://qgis.org/ubuntu `lsb_release -c -s` main"
Update your repository information to reflect also the just added QGIS one:

sudo apt-get update
Now, install QGIS!

Note: add ‘qgis-server’ to this line if you also want to install QGIS Server:

sudo apt-get install qgis qgis-plugin-grass qgis-server