Go find the health bar in the scene. Expand it so you can see Background and Fill gameobjects (fill should be under Fill area). Fill is your health bar, Background is what you see when your health bar is drained/gone. Replace with whatever image you want. Make sure you 9 Slice the sprite, or else it just stretch the image.
If you want to do Zelda Hearts, there are alot of tutorials on youtube. It's a bit more complicated because you'll want it to be dynamic, since you don't know the max hearts you want in a scene.