When tuning the pitch reed you must first set your self up for the job you need quite, solitude, a bench to work on, a magnifying glass, and a chromatic tuner.
proceed with methodical caution, with a felt tip mark the reed you wish to tune and with the reed plates in the comb play the reed several times to get a good accurate reading on the tuner.
then literally one scrape at a time begin to raise or lower pitch as you require using a reed file.
if you tune the reed too far over perfect pitch just scrape back one in the opposite pitch.
it is a distinct advantage to gain experience in altering pitch to a reed by investing in a cheap harp just for this purpose, once you have the confidence to understand the effects of the scrape on a reed you will take control of your musical instrument along with a clear understanding of the mechanics of playing a harmonica