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

S-Grade:
To exchange an Upper Armor piece, you need: 5 S-grade Spheres.
To exchange Helmet, Upper Armor and Lower Armor piece, you need: 3 S-grade Spheres.
To exchange Gloves, and Boots, you need: 2 S-grade Spheres.
To exchange Shields, Sigils, Accessory, you need: 1 S.
To exchange Weapons, you need: 4 S-grade Spheres.

Dynasty:
To exchange an Upper Armor piece, you need: 5 S-grade Spheres.
To exchange a Helmet and Lower Armor, you need: 4 S-grade Spheres.
To exchange Gloves, and Boots, you need: 3 S-grade Spheres.
To exchange Shields, Sigils, Accessory, you need: 2 S.
To exchange Weapons, you need: 5 S-grade Spheres.