To lower the pitch of a reed, remove metal from near the reed base. Alternately, to lower the pitch of a reed add lead-free solder to the tip of the reed. You are removing metal from the flat surface of the reed, not at the edges, which would increase the air gap around the reed and cause air loss in the chamber. See the picture in the Reed Gapping section. Various tools can be used for removing metal from the reeds.
Different people have their own preferences.
I prefer a dremmel-type rotary tool with a hard rubber disk, which results in a smooth surface, and doesn't remove material as fast as a stone or burr. I suggest removing small amounts of metal from a large area, minimizing any gouging or scratching or the reeds. Before attempting to remove metal from a reed, you need to support the reed so you don't push it through the slot. A thin shim like a .003 spark plug gapper works well, as does a razor blade. Be careful not to push the reed sideways in its slot, or the reed won't vibrate freely.