Like other legendary items, legendary upgrades are acquired using the Mystic Forge. You’ll need to provide several gifts, including one that requires new materials unlocked by a global event.
On July 30, citizens will be called on to trade in their piles of lucent crystals to crafting discipline grandmasters in exchange for Satchels of Loot, with the goal of forcing the knowledge out of the Mystic Forge. Every donation progresses a global community goal. Once the goal is complete, the ability to create new materials for legendary upgrades will become available to every discipline that can craft upgrades.
https://www.guildwars2.com/en/news/harness-the-power-of-legendary-upgrades/