How to update RPi OS

Hi,
I may be missing something obvious, but how do I update the OS of my RPi?
The console has only a minimal command set.

I started with HASS.IO image “hassos_rpi3-2.12.img.gz” a while ago.
Now updating HA to 0.100.3 it warned that I am running an old version of pip so I assume OS update is needed.

Br Kari