Different tricks to hack the pi

sudo nano /etc/apt/sources.list

deb http://ports.ubuntu.com/ubuntu-ports bionic-updates main

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 3B4FE6ACC0B21F32

sudo nano /etc/apt/preferences.d/99bionic-updates

Package: *
Pin: release a=bionic-updates
Pin-Priority: 1

sudo apt update

sudo apt install firefox

https://www.raspberrypi.org/documentation/raspbian/updating.md

To sum up you use the apt command but before you change manually the repositories debian package configuration.

To upgrade, first modify the files /etc/apt/sources.list and /etc/apt/sources.list.d/raspi.list. In both files, change every occurrence of the word jessie to stretch. (Both files will require sudo to edit.)

sudo apt update
sudo apt -y dist-upgrade

sudo apt full-upgrade