Question

You can change the time signature by simply adding it from the palette. But if you mean you actually created a ton of triplets and now you want them "untripleted", unfortunately that's not possible. There is, I suppose, some chance that with enough fiddling about in a MIDI sequencer, you could make the change there - so export to MIDI, load there, edit, then reimport. Depending on how much music you already are working with, that could possibly be more work than re-entering.