You can't. Unless you have gamesharks code or and action replay then your out of luck
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you need to get evee and give it love in the morning to get Espeon and at night to get Umbreon
Yes it is possible to get Espeon and Umbreon in Pokémon FireRed but you cannot physically evolve Eevee into either Espeon or Umbreon in Pokémon FireRed due to there being no time cycle in the game so you will need to evolve Eevee into Umbreon or Espeon in Pokémon Ruby, Sapphire or Emerald and then trade it over to FireRed.
The evolutions of Eevee are: -Flareon (fire) -Jolteon (electric) -Vaporeon (water) -Espeon (phsycic) -Umbreon (dark) -Leafeon (grass) -Glaceon (ice)
You can evolve it any time, and I mean ANY TIME as long as you have the stone for it... Fire Stone= Flareon Water Stone= Vaporeon Thunder Stone= Jolteon Moon Stone= Umbreon Sun Stone= Espeon I believe that's correct, but I'm not certain on the Umbreon/Espeon deal but yeah.
fire stone = flareon water stone = vaporeon thunder stone = jolteon (For fire red & leaf green) sun stone = espeon moon stone = umbreon have fun :D