Trade it to ruby,sapphire or emerald have them evolve your evee at night then give it back to leafgreen.
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In ANY GAME!! If you use a moonstone, i think
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
To make Eevee evolve into an Umbreon, you have to level it up with high happiness at night.
Evolve it at 12 pm
Eevee doesn't have a set level to evolve into Umbreon. All you have to do to evolve it is max out its happiness at night. ~KKMG1