The Statue's Bargain
The Horned Statue is just north of Firly Pond and is technically the quest giver, so you can head directly to the Horned Statue if you wish. However, for help finding the statue, first look for Teebo. Teebo is a child available near the western entrance of Hateno Village roughly between noon and 7 p.m. Talk to him and he will lead you to the statue.
Speak to the statue for the first time and it will take a Heart Container or Stamina Vessel from you. Speak to the statue again and you can get your Heart Container or Stamina Vessel back, which will complete the quest.
From this point on, you can sell the statue either a Heart Container or Stamina Vessel for 100 rupees and then buy back either of them for 120 rupees, costing you a net 20 rupees to exchange them. This is useful for when you need more stamina than you need life, or vice versa. In particular, if Link has a lot of Stamina Vessels, but is trying to acquire the Master Sword. He can temporarily exchange his Stamina Vessels for Heart Containers so that he can acquire the 13-required heart containers to get the Master Sword.
You persuaded the horned statue to return your container. Now you can exchange containers for a small cost.
The horned statue will buy a Heart Container or Stamina Vessel for 100 rupees and sell your choice afterward for 120 rupees.