Please refer to Blacksmith Pushkin in the Town of Giran to exchange a common item of B-grade for a rare item.

To exchange a Full Armor piece, you need: 3 B-grade Spheres.
To exchange an Upper Armor piece, you need: 2 B-grade Spheres.
To exchange Weapons, Accessory, Shields, and other Armor pieces, you need: 1 B-grade Sphere.