There are videos online which detail this, but the key is, the longer the red, the deeper the note. Problem is, of course, that it is easy to sharpen a note and hard to make it flatter.
And yes, you have to open up the harp to get to the reeds, but that is just two screws. Or, entry level harmonica are like $5 and my Marine Band was $20, so they are very replaceable.
I go to Adam Gussow (KudzuRunner on YouTube) for all my harmonica questions, including how to tune them.