To evolve Eevee into Leafeon, you can use the nickname "Linnea". Once you have renamed your Eevee, you should quit and reload the app to double-check the name change has taken place. Its worth noting that this method only works once per evolution. Alternatively, you can evolve Eevee into Leafeon by using a Mossy Lure Module on a nearby PokeStop and hitting the Evolve button while in range.