-- Lean 4 proof step from Mathlib.Geometry.RingedSpace.PresheafedSpace.HasColimits (goal)
Goal:
J : Type u'
inst : Category.{v', u'} J
C : Type u
inst_1 : Category.{v, u} C
inst_2 : HasColimitsOfShape.{v', u', v, v + 1} J TopCat
F : J ⥤ PresheafedSpace.{v, u, v} C
j : J
⊢ Eq (α :=
    Quiver.Hom (V := Presheaf.{v, v, u} C (colimit (F ⋙ forget C)))
      (unop (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf)))
      (unop (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf))))
    ((opEquiv (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf))
            (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf)) :
          (a :
              op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf) ⟶
                op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf)) →
            unop (op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf)) ⟶
              unop (op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf)))
        (Quiver.Hom.op
          (map (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.map (𝟙 j)).c ≫
            Iso.inv (Pushforward.comp ((F ⋙ forget C).map (𝟙 j)) (colimit.ι (F ⋙ forget C) j) (F.obj j).presheaf) ≫
              Iso.hom (pushforwardEq ⋯ (F.obj j).presheaf))) :
      Quiver.Hom (V := Presheaf.{v, v, u} C (colimit (F ⋙ forget C)))
        (unop (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf)))
        (unop (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf))))
    ((opEquiv (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf))
            (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf)) :
          (a :
              op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf) ⟶
                op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf)) →
            unop (op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf)) ⟶
              unop (op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf)))
        (𝟙 (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf))) :
      Quiver.Hom (V := Presheaf.{v, v, u} C (colimit (F ⋙ forget C)))
        (unop (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf)))
        (unop (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf))))
have a : ∀ (U : (Opens (↑(colimit (F ⋙ forget C)) : Type v))ᵒᵖ),
  Eq (α :=
    obj (unop (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf))) U ⟶
      obj (unop (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf))) U)
    (NatTrans.app
      ((opEquiv (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf))
            (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf)) :
          (a :
              op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf) ⟶
                op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf)) →
            unop (op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf)) ⟶
              unop (op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf)))
        (Quiver.Hom.op
          (map (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.map (𝟙 j)).c ≫
            Iso.inv (Pushforward.comp ((F ⋙ forget C).map (𝟙 j)) (colimit.ι (F ⋙ forget C) j) (F.obj j).presheaf) ≫
              Iso.hom (pushforwardEq sorry (F.obj j).presheaf))))
      U)
    (NatTrans.app
      ((opEquiv (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf))
            (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf)) :
          (a :
              op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf) ⟶
                op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf)) →
            unop (op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf)) ⟶
              unop (op ((pushforward C (colimit.ι (F ⋙ forget C) j)).obj (F.obj j).presheaf)))
        (𝟙 (op (obj (pushforward C (colimit.ι (F ⋙ forget C) j)) (F.obj j).presheaf))))
      U)
Tactic: refine NatTrans.ext (funext fun U => ?_)