You can't. It evolves by the time of day. Since there isn't a clock, eevee won't evolve into espeon OR umbreon.
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so far, you cannot evolve eevee either to umbreon or espeon on FireRed because on FireRed and LeafGreen, there is no clock system. You must trade eevee to Emerald for evolving to Espeon or Umbreon
Evolve it from eevee
Ive hear my friends say that in Pokemon yellow you can evolve eevee in a espeon with a moon stone
give it a thunderstone
You will get an eevee egg. That you can evolve into an eevee or the other eevee evolutions.