Most armor within The Legend of Zelda: Tears of the Kingdom can be upgraded. In order to upgrade a specific piece of armor, your must have unlocked one of the four Great Fairy locations within Hyrule.
Many Landmark locations in Zelda Tears of the Kingdom, spanning its Surface, Sky, and Depths. A 100% map completion in Zelda: TotK is one monumental task, taking well over 100 hours to accomplish.