Firmware update 2.1.3 is a software patch released by Canon to address several issues and improve the overall performance of the EOS 5D Mark II. This update was released on February 2012, and it's aimed at fixing problems related to the camera's autofocus, metering, and playback modes.
Visit the Canon Support Site and enter "5D Mark II." [1] canon 5d mark ii firmware update 2.1.3 download
Yes, you can install an older firmware version, but it is rarely necessary. You would need the older .fir file and follow the same procedure. Firmware update 2