First of all; backup your important data. You can do this from the terminal with a simple cp command. Usually your data and personal settings are saved in your home folder (e.g. /home/username/). You can use cp -a to backup your home folder to an external drive (see man cp). Preferably, the external drive should have a Linux file system too (e.g. ext4), otherwise you will lose your file permissions.
If your home folder is actually on a separate partition (I believe the Ubuntu installation does this by default now), then you can easily revert to Ubuntu 12.04 without losing your data and personal settings. To check whether your home folder is on a separate partition, run the command df from the terminal. If "/home" appears in the "Mounted on" column of the output, then it is indeed a separate partition. To revert to Ubuntu 12.04, just reinstall it using the official Ubuntu 12.04 installation disc. During installation, tell it to replace Ubuntu 13.10, but keep and reuse the existing home partition (do not format the home partition). If you are lucky, it will appear as an option during installation at the partitioning step. If not, you will have to select manual partitioning (see question 285212). Again, do not format the home partition.
If your home folder is not on a separate partition, you can still install Ubuntu 12.04 using the official installation disc, but you will lose all your personal data in the process (because it is on the same partition as Ubuntu itself). But you can easily restore your data and personal settings with the backup you created. Just copy everything back into your home folder (e.g. /home/username/) after installation.
Since this is a clean install, you will have to reinstall any software that was not installed by default. But that is easily done via the Software Center.
