Why do you want to upgrade it to 12V? To get increased performance you have to overvolt the motors (the voltage going to the motors is the 12V battery voltage and is not stepped down from 12V to 5V). The problem is that a lot more heat is produced. If you cannot get rid of this heat, you will burn out your motor.
Anyways, if you want to overvolt your motors, all you do is stick a 12V->5V regulator between the battery and electronics and let 12V run straight to the motors. At some point though, the 12V going to the motor will pass through some of the electronics used to control the speed, you have to make sure the 12V running through these circuits won't damage them.
I am going to repeat this again (in case you don't speak English and do not understand what I mean). Add a 12->5V regulator for the electronics. And leave the motors connected to the 12V battery. Make sure the speed control electronics will not be damaged by 12V motor current running through them.