Open a Terminal
Firstly you have to update the package list. Type:
sudo apt-get update
Then, when that has run, you need to upgrade your machine:
sudo apt-get upgrade
Then to throw the wrapping paper away(!):
sudo apt-get clean
That’s it, although after a kernel upgrade (ie where a “linux-image” package has been upgraded), it’s a good idea to restart your system so the new kernel can be loaded.