Reduce 6 volts to 5 volts?
First, you have to know, what current your device will draw.
I'd personally use a DC-DC with low voltage drop or a simple LDO if the current isn't too high. There's also an option, to use diodes.

option A: Buy over the internet or create your own low drop DC-DC buck converter, which will hold the power dissipation for current across your device.



option B: Use LM1117 5.0V if your current won't exceed 1A and you can have 6.3V on the input.



option C: Use 1N4007 and 1N4148 diodes in series (same direction), where the 1N4007 will cause ~0.7V drop and 1N4148 will cause ~0.3V drop.



option D: if the current is small, you can use just a simple voltage resistor with proper ratio!