Before using any one of the following method make sure, you have a backup of important data.
Method # 1
First edit your /etc/apt/sources.list file and replace all word that read as breezy to dapper as root user (type any one of the following command) :
gksudo vi /etc/apt/sources.list
gksudo gedit /etc/apt/sources.list
Save the file.
Type following commands to upgrade from breezy to dapper:
sudo apt-get update
sudo apt-get -y dist-upgrade
Method # 2
Alternatively, Upgrading procedure is extremely simple if you use GUI tool called update-manager to upgrade from previous version. Open terminal and type the following command:
gksudo update-manager -d
Click on check button to upgrade.
Ubuntu Dapper on the Apple Macbook
Check out instructions on installing on Apple's MacBook.