To upgrade accessories, you'll use the Fusion system. Fuse lower-star accessories into higher-star ones to increase their level and unlock their full potential. Once an accessory reaches max level, you can re-roll its secondary stat. This is a critical endgame mechanic. Use Spirit Orbs or Re-roll Brushes to try and get the desired secondary stat (e.g., 30% Attack for NAD, 30% Spiritual Pressure for SAD). It can be a resource-intensive process.
