I do it “manually” in the BB Midi Editor because I’ve gotten pretty good/fast at it. It’s (usually) pretty quick UNLESS your song has a lot of parts or a lot of bass notes (most of the songs I work with only use something like 6-12 notes). If I’m transposing UP, I start with the highest note, and move every instance of that note up the appropriate amount, repeat with the next highest note. If I’m transposing DOWN, I start with the lowest bass note (this avoids confusion between the “old” and “new” notes), repeat with next lowest note. Check your work by listening (because it’s very easy to make mistakes), save as a new song, and include the new key in the song’s name. Of course, there are other ways to accomplish this, perhaps more to your your liking - YMMV.
As to Persist’s mention of using the Editor causing bass audio quality, I’ve never had this issue - and I can’t even imagine HOW shifting MIDI data could affect audio quality - but I’ve spent a lot of hours in a state-of-the-art studio (NOT my own), and I’ve learned that when things go wrong in the digital realm, they sometimes do so in VERY unexpected (and sometimes very-hard-to-diagnose) ways. That might be partly why I like editing BB files the old-school way: As rough as the BB Editor is, it’s simple, consistent, and stable. And it’s the devil I know:)