Itunes will prompt you to upgrade. As a rule, never upgrade unless the Dev Team says its all good. Apple uses its upgrades to fix very minor issues and of course to close up the exploits and kill jailbreaks.
3.13 is a JB killer masked as a battery meter improvement.
Downgrading to 3.12, assuming you were never JB on 3.12, is not possible as far as I know.
If you JB on 3.12 and had your hashes saved on Cydia's server, then you have options.
Check out the following sites for lots of info.
Dev-Team Blog
iPhone Modifications
Bottom line is you're probably out of luck.