One of the problems harmonicists face, except of handling their instrument (I mean their harmonica), is playing with a fine tuned instrument. I 'll give you pieces of advice on what I've already experimented (learning by myself). Do not hesitate to notify me with all your tips, exchange a good address or ask for more details.
To be clear about it, there are two things to be done :
To lower the note, there are two ways. Either you file the foot of the blade (not advisable!), or you can weigh its head.
I advise against the first solution because, on the one hand, it might weaken the blade, and on the other hand, if you lower the note too much, you'll have to raise it by filing the head...and if you file too much, back to your initial problem... Needless to say that it can turn to a mess...and the blades might become too weak. Nevertheless those who dare can read the following explanations and instructions.
The second solution (weighing the head) seems the better one, since you don't weaken the blade (no metal off) and you can always cancel this fixing by taking the weight away... Indeed!!! My secret is to use superglue.... Here's how I do.... In every case you should first try with an old harmo...
At first, be cool because you'll have to be very meticulous... Stay zen and bring out your nimble fingers !
For my work I use the Korg AT-12 tuner. It's not the cheapest one but it's essential for a good work...
Select the blade you want to lower ; find its height with the tuner, then unscrew the hood of the harmonica : aspirated notes are obtained with the visible blades, blown notes with the blades on the "inside" side of the plates. If I want to lower a blown note, I dismantle the plate and put it back on its wrong side so that the blade can be easily fixed. Put a Bristol board under the blade, then put a drop a glue on the end of the strip. You should not overflow (in that case the Bristol board may prevent from any damage) ; the drop of glue should be put on the very centre of the blade, avoid contact with the edges (see the following instructions).
Roughly speaking litres of glue are not necessary... Keep it dry one hour or two (put the harmo under a lamp to save time but not too close because of the plastic...). Blow or breath in into the hole of matter and watch at your tuner. At this stage, the rest is pretty easy...
For those who dare and have a bigger account than mine, they will be able to purchase a new harmo (indeed you're not a state servant nor temping in entertainment), you can try the file option...
To lower a note you must file the foot of the blade : not anyhow of course... What's best is to file very flat and diagonally, not really on the foot.
Let's see for the second case
You should need it for a special tuning or simply to ajust a note. The fifth aspirated hole on a 10-hole Richter system is particularly sensitive and tends to lower, especially if you play wildly. To be blunt about it, if after the tuning the note keeps on lowering, the metal might be spun. In this case you should change the blade or the harmo... To change the blade, a friend of mine can help but I never tried. Please contact me to get in touch with him .
My unique suggestion would be to file the blade. The main technique remains the same but this time you have to file the head of the blade.
CAUTION !!! You shall never file the blade shorter ; you should instead file on the thickness. Always file diagonaly and flat...
You're now ready to save money. I've a few friends, harmonicists, with dozens of harmonicas which have lost a note...It's time to try out...
You should also want to fix your harmo in order to make it play unexpected sounds, maybe just for fun, to solve a problem (how to play this piece all in G and with this stucking C sharp with your 10-hole harmo ?) or because you want to try new scales... Of course the technique to raise or lower the note remains the same...
Among fine tuning...
* Lower each third of a 10-hole harmo without fixing the aspirated notes
* Raise of a half-key the F on a C harmo (fifth aspirated hole)
* Lower the B of a half-key on a C harmo (seventh aspirated hole)
The rest is a question of imagination. I'm not familiar with musical theory but by playing a fixed harmo I got the inspiration for some pieces which don't sound "so worse"...