There are no easy ways. There are two methods that I would use.
- Make a copy of the entire drumkit. Rename the kit. Adjust the volume of the pieces that would want to adjust within that drumkit. Then save the drumkit. I have a tutorial on renaming and editing a drumkit.
- The other method is to open the individual midi file that make up the song in a DAW, and then adjust the velocities (not the volumes), of the drums that you want to change. BB does not respond to midi volume, but it does respond to velocity.