Never heard of a neutral key signature. Can you elaborate? Do you mean, no key signature and all accidentals marked?
You change key signatures with either the selection tool flyout, the key signature tool, or the dialogue box. In this example, m. 1 is in Ab, but the checkbox for "no key signature and all accidentals marked" is checked in the KS dialogue box. In the second measure, I simply chose the Ab key signature.