1).Undo unfolding
2).Go to 3d menu and choose "edit mode",then "edit model".You can swap/delete/divide lines within the model,or change its shape by moving "little black squares" or Virtexes located on each "cross-section" of polygons.
Personally I used this feature to customize the waist of the torso I'm planning to use as a foundation for my costumes.Otherwise I would never be able to wear it,since 3d game models don't have realistic dimensions at all (exaggerated V-shapes etc.).I then stretched the area below the pecks/chest muscles in W10 3d builder to make it match my torso height.
3).Unfold when you're done customizing.
I hope this helps