For general instructions on tuning your harmonica, start here: https://harp.andrewzajac.ca/tune
This video only covers one technique - adding solder to the tip of a reed. As a rule, you can lower the pitch of a reed by five semitones and you can raise it by two. You can go much further but you will run into problems with response and technique if you do.
Take material off the tip to raise the the pitch of a note. Take material off the base or add material to the tip to lower the pitch.
If lowering more than a semitone, maybe it's best to add weight for most of the drop in pitch? Here I drop the pitch one full tone. In the process, I drop the pitch much further down and then tune it back up.