Hi there,
The easiest way to do so is as follows:
1. Stop the BE services, including the SQL service.
2. Grab manual copies of the Data and Catalogs folders in the installation directory.
3. If anything goes wrong, you would then just reinstall BE 2012 and follow normal procedures to get it back online (stop the BE services, rename the new Data/Catalogs folders, and copy the backed up folders into that installation directory before starting the services up again).
Also, during the upgrade, BE 2014 checks to see if you have any issues and advises you accordingly before proceeding. So you have a means to back out of the upgrade.
My advice would be to consider stopping any AV installed on the appliance before doing the upgrade. I've had McAfee causes issues during the SQL upgrade where it blocked the process, and erased the BEDB!
Thanks!