sudo apt-get updateUnfortunately, when I upgrade my Maverick to Natty, those two don't work. Then I googled it and find simpler method. Here we go:
sudo apt-get dist-upgrade
# Look for Update Manager menu. Then click Install Update button (it is located in bottom left of dialog box).
# Wait for approximately three hours (this is based on your internet speed)
# Restart your PC.
Welcome to the new look of Natty. Enjoy!