Equipment upgrading allows you to change the grade of weapons, armor and accessories. In order to do that, you need to collect all necessary materials.
But unfortunately shields and sigils cannot be upgraded. Weapons can be upgraded after +7 enchant level, armor - after +6 enchant level, circlets, Dragon Skin Shirt, Kaliel's Bracelet, La Vie En Rose's Brooch and Artifact Books - after +5 enchant level.

Upgrading Equipment

Item for upgrade | Result
Apocalypse (blessed) | Specter Weapon
Twilight (blessed) | Seraph Armor
Specter (blessed) | Helios Weapon
Seraph (blessed) | Eternal Armor
Helios (blessed) | Krishna Weapon
Eternal (blessed) | Leviathan Armor

Required materials

Weapon | Armor
Blessed weapon | Blessed armor
R-grade Gemstone | R-grade Gemstone
R-grade Crystal | R-grade Crystal
Crystal of Determination | Crystal of Protection

Accessories

Accessories | High-grade Resources | Top-grade Resources
Circlets | Noble Upgrade Stone | Radiant Upgrade Stone | Dragon Skin Shirt | Noble Leather Upgrade Stone | Radiant Rind Upgrade Stone | Kaliel's Bracelet | Crystal of Dawn | Crystal of Dawn | La Vie En Rose's Brooch | Crystal of Dawn | Crystal of Dawn | La Vie En Rose's Brooch | Crystal of Dawn | Crystal of Dawn