While exploring Hyrule, you should come across four large flowers that ask for Rupees. These are Great Fairy Fountains, and they are used to upgrade your Armor and Jewelry. After paying a base fee in Rupees, the Great Fairy will reveal herself and offer her services. The first fairy asks for 100 Rupees, the second asks for 500 Rupees, the third asks for 1,000 Rupees, and the fourth asks for 10,000 Rupees.
To upgrade a piece of armor, you must bring a specific number of materials along with item itself. Enhancing and equipping all three pieces in a set of armor will often give Link a special set bonus, though that’s not always the case. Armor can be upgraded a max of four times, but only after finding all four Great Fairy Fountains.
West Necluda, east side of Kakariko Village.
Tabantha Frontier, west of Tabantha Bridge Stable.
Akkala Highlands, east of Lake Akkala
Gerudo Desert, under the Great Leviathan skeleton.