use an lm317 regulator ,and connect a potentiometer to vary the output voltage setting it to 6.5 volts (do this method if you have a multimeter)
just google out lm317 , and check out the images , you should be able to find the circuit diagram
also the 5 diode solution given by neal was creative and strange ,the diodes that i have has a cutin voltage from 0.5 v to 0.7 v but it was worth a thought provided neal has got diodes which drops of 0.7 volts precisely , at a wide range of loads connected to it