The procedure to upgrade to the "Mixed Firmware" is the same as the "standard" one.
Note, however, the new firmware needs new IVONA TTS 4. I do not know what version of TTS you have now. The only help I can provide is ... connect your device to a PC, zip the ".tts" folder from the "Internal Storage" and provide that file. I can try to unpack it on my device and test whether it works with the new firmware (which I use).