Prior to Ubuntu 12.10 you can able to upgrade your Ubuntu OS with the help of Ubuntu Alternate CD. But with the release of Ubuntu 12.10 Steve Langasek from Canonical announced on August 27th, a proposal to drop the Alternate CD ISO images starting with the upcoming Ubuntu 12.10 (Quantal Quetzal) operating system. On August 30th, the Alternate CDs were gone from the daily builds and will not be available at the launch of Ubuntu 12.10 Beta 1. For more information your can go through following link

Proposal to drop Ubuntu alternate CDs for 12.10

In case you want to upgrade from the disk which you have downloaded from the internet then follow below steps

Step 1. Boot with Ubuntu 12.10 Disk

Step 2. Select Install Ubuntu Option

Step 3. On the installation type screen choose Upgrade Ubuntu 12.04.1 LTS to Ubuntu 12.10

In this option "Document, music and other personal files will be kept. Installed softwares will be kept where possible. System-wide settings will be cleared."

Kindly Note that as a safer side kindly take the backup of your data.

In case you want to update your Ubuntu OS using update manager then follow below post

How to Upgrade / Update Ubuntu OS to Ubuntu 12.10 (Quantal Quetzal) from 12.04, 11.04, 11.10