Log in to your Raspberry Pi

Edit the sources package:

sudo nano /etc/apt/sources.list

Replace SOME of the text on the first line

Replace this:

deb http://mirrordirector.raspbian.org/raspbian/

With:

deb http://mirror.aarnet.edu.au/pub/raspbian/raspbian

Save and Quit (Ctrl-X, Y, Enter)

Run the update command:

sudo apt-get update

Run the upgrade command:

sudo apt-get dist-upgrade

This will take a while!