One of the oldest features of every The Legend of Zelda game includes the Great Fairy Fountain which allows Link to upgrade his armor. Here is a Legend of Zelda Breath of the Wild guide that would show each of its location in the game.
After obtaining the Ancient Armor Set through a Legend of Zelda Breath of the Wild guide posted previously, Link has the option to improve it. Once improved, each piece would have four times the defensive stats it had before.
Nintendo had carefully hidden each of the Great Fairy Fountain location and this Legend of Zelda Breath of the Wild guide would reveal all of them. However, Link would need a certain amount of rupees as well as specific items in order for the upgrade to succeed.
According to VG247, Link has to awaken four great fairies that would upgrade his armor for him. Each fairy encountered would add to their collective power, making each armor enhancement better.
The fist great fairy would require 100 rupees, the second 500, the third 1,000 and the last with 10,000. After awakening all of the great fairies, Link can upgrade all his armor up to the highest level, provided that he meets the requirements.
Great Fairy Cotera
The first one is Great Fairy Cotera and Link can encounter her in Lanayru. Simply climb to the Ta’loh Naeg Shrine from Kakariko and by its right, a path will exist, leading Link into the woods.
Take the left-hand path at the fork of the woods and he will encounter an enormous flower. Climb up the petal steps up to the flower and Link can interact with Cotera. Pay up the required amount and he will unlock the first level upgrades.
Great Fairy Mija
The next one, Great Fairy Mija, rests in Akkala Citadel Ruins and to the right of Lake Akkala. Go below the circular island in the middle of the Lake and he can find the second Great Fairy Fountain. Interact with Mija and pay 500 rupees in order to unlock the second level enhancements.
Great Fairy Kaysa
Link can locate the third great fairy, Kaysa, near the Tabantha Great Bridge and Nero Hill. Climb the Tabantha Tower and look south and locate the bright flower. Once seen, simply glide down, interact with Kaysa and pay 1,000 rupees for the next upgrades.
Great Fairy Tera
The last great fairy, Tera, hides in the cruelest area in the game, the Gerudo Desert. From there, look for the area marked as the Dragon’s Exile and upon reaching that, Link would encounter the Gerudo Great Skeleton. The last Great Fairy Fountain would rest there and interact with it would prompt Link to pay 10,000 rupees for the last and final enhancement.