hey. the general key of the piece is typically something that is arrived at fairly early on in the writing process - sometimes a deliberate choice is made, other times the key emerges inadvertently via choice of samples, drum sounds etc.
the key of the piece is not generally something that one would change on a whim toward the end of the process and i've not seen any global-type facility on ableton live that would serve to turn a major piece into a minor. changing the root (tonic) note is something different, but i assume you wish to turn a major key into a minor one.
if that's your goal, then you'd most likely have to re-write various parts of the tune - basically changing all the major thirds to minor ones (relative to the key) - so if your key is C major, locate all E notes and change them to D sharps. bit of a crude solution - the ideal is to be aware of what key you're writing in whilst you're writing it - but that should get you on your way. peace.
