Ah, the real question is "what do you wan to do with the 6V?"
If the answer is "I want to power a device that requires 6V" then the strict answer is "Use a boost DC-DC converter". A simpler answer might be "Just use the 5V, it will probably work anyway".
If the answer is "I need a 6V signal and all I have is 5V" then the answer is to use a couple of transistors to create an amplifier (powered from a voltage significantly higher than 6V).