Are they just a bit tarnished? If so, some 0000 steel wool will clean them up. You can either do it along the length of the fretboard (up and down the neck), and you don't need to cover the board, or across the width of the neck, in which case you will need to cover the fretboard (some blue painters tape either side of the fret works perfectly, and just move it to the next fret as you finish. You can also use masking tape, but you should stick it to a t-shirt or something first, just to take a bit of the stickiness off)