You’ll need to get yourself some bass samples to replace the bass in the kit. My roll your kits have several basses in them.
It helps to have a bit of theory background, but you can get there without it. Lets say, you want to lower the bass by one full note because its a bit too high for you. Every G, should be an F, for example. What you are going to do is open up the drumkit in BBMO, copy it, and give it a new name, maybe “kitname”-my edit.
Then begin to replace every bass wav file in the kit in BBMO with a corresponding bass bass file that is one full step lower.
G3 > F3
Gb3 > E3
F3 > Eb3
E3 > D3
Eb3> Db3
D3> C3
Db3 > B2
C3 > Bb2
B2 > A2
Bb2 > Ab2
A2 > G2
Ab2 > Gb2, etc.
You won’t be able to go below the lowest note in the available set of samples. For those notes, first transpose them up one octave and then replace with the appropriate wav for the transposed note.