A message will appear in the notification bar at the top of your screen when the update is available. You need to make sure that you are connected to a Wi-Fi connection to perform this update and follow further on screen instructions.
Step-by-step screenshots are below, as with all upgrades make sure you backup all your data first.
I love tech things, especially the way they infiltrate our lives without us even realising. I don’t like having to explain how a gadget works to every member of my family though!