In this schematic is presented a simple 12 Volts to 6 volts converter circuit that is built with the well-known 7805 voltage regulator or the LM309 IC. You can use 7806 as well, but this circuit comes in handy when you do not have a spare 7806.
Schematic of the 12 V to 6 V converter circuit
You can get out a different voltage, for example 7.5V and in this case you have to use 3 or 4 diodes at the GND pin of the IC. The maximum output current will be 1 amp and you need to use a good heatsink.
The actual voltage might be different, for example using 2 diodes as shown in the schematic can give an output voltage of 6.5V and using one diode will result in a 6V output. It is better to assemble the circuit on a protoboard (breadboard) and test.