Q. How to cite a critical edition of a music score.

You can cite critical editions just like you would other scores. The only difference would be you would definitely have an editor that would go after the composer and title. You might also have an author of a preface or something like that who you may want to include. 

Here is the Chicago Manual of Style section on citing musical scores: https://go.umd.edu/TzS (login required)

The first example they give there, the Verdi example, is actually a critical edition, although it doesn't say so. 

The only weird hiccup with these things would be if you are citing specifically a standalone critical commentary that is part of a series, in which case you probably would cite that more like a book. For all of these it is usually best to just look at the title page in the score and let that be your guide.