To update software in Raspbian, you can use the apt tool in a terminal. Open a terminal window from the taskbar or application menu:
First, update your system's package list by entering the following command:
sudo apt update
Next, upgrade all your installed packages to their latest versions with the following command:
sudo apt full-upgrade
Note that full-upgrade is used in preference to a simple upgrade, as it also picks up any dependency changes that may have been made.
Generally speaking, doing this regularly will keep your installation up to date for the particular major Raspbian release you are using (e.g. Stretch). It will not update from one major release to another, for example, Stretch to Buster.
However, there are occasional changes made in the Foundation's Raspbian image that require manual intervention, for example, a newly introduced package. These are not installed with an upgrade, as this command only updates the packages you already have installed.
The kernel and firmware are installed as a Debian package, and so will also get updates when using the procedure above. These packages are updated infrequently and after extensive testing.
