Yes, you can download past versions of the camera firmware from the
Support Page on the Blackmagic Design website. Just make sure you tick "Include Legacy".
If you want to roll-back from 1.3 to 1.2.1 for example, just uninstall 1.3 on your Mac/PC, install 1.2.1, then run the
Blackmagic Cinema Camera Utility. As Aaron said - there are instructions in the camera's manual if you get stuck.