Install it on a USB key. If you want to try the upgrade but have a perfect backup of 12.04 LTS, you can image your drive (depending on resources) with the "dd" utility. WARNING: be very careful with this. You can easily destroy things with dd.

From the USB install;

  dd if=/dev/{ubuntuDrivePath} of={someOtherDrive}/{backupFile}.img

This creates a 1 to 1 representation of your drive. For example,

  dd if=/dev/sda of=/media/data/backups/workstation.img

You will of course need to use an additional drive to store your backup.

To restore, boot into USB again, and execute:

  dd if={yourBackup} of=/dev/{ubuntuDrivePath}