With the power off, examine your board and look for a little white "block" with a round part in the top and it will have either a + shape groove or a - groove. (IN otherwords it will look either like the top of a phillips screw, or like the top of a flathead crew). This should be your volume knob.
Now you can either adjust it there and then power on to see how loud it is, or you can power on and very carefully adjust it while it's playing sound, but just be careful not to slip and touch anything with the screwdriver. There's no risk of big deathly shock if you touch something on the board (it'll be a mild shock if any) but you do risk damaging parts.
I actually use my fingernail to adjust the volume. Less chance of a tool slipping and touching parts.