There is no way to directly remap these buttons. They are hardcoded into the game (And baked into the graphics, but that's besides the point).
If you're using a mouse with custom mapping software (Microsoft mice come to mind because I have one and it works for this specifically), you might consider creating a game profile for Notrium that remaps U and B to buttons 4 and 5.
Failing that, if you have any experience modding Notrium, you may be able to write a script that triggers the action for you when you press a different button. U and B will still work, which may/not be what you're looking for. On top of that, I haven't played Notrium since I-don't-remember-how-long, nevermind modding it, so I'm not sure it was actually possible or where I saw it done already. (This is really a last-resort solution.)