The short answer would be:
1. Make a backup.
2. Change all occurrences of the word natty
in the file /etc/apt/sources.list
sudo apt-get update
sudo apt-get dist-upgrade
while hoping that nothing brakes.
You better do not upgrade to 12.10, it is unsupported, either go for 12.04 (as described above, supported until 2017) or 14.04 (supported until 2019, upgrade to 12.04 as described above and then to 14.04).