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You can download the installer here: https://help.ubuntu.com/community/GettingUbuntu. After downloading the installer, save it onto a bootable DVD or flash drive. Although Ubuntu can be used just by running it from the DVD or flash drive installation disk, installing it on your computer will wipe out all the data on your hard drive. Back up all the important files and documents before you proceed installing Ubuntu. If you're using a flash drive, plug it into any USB port on your computer.
Download an Ubuntu installation disk. Back up all the files of your computer. Insert the DVD installer into the CD drive. Restart your computer.