Question

I want to evolve my eevee into Umbreon.

Answer 1

you cannot evolve your eevee into umbreon or espeon in firered/leafgreen because they need not only happines/friendship but also they need time to evolve (day espeon/nigth umbreon) and since in kanto there were no time events you can't, the best form to get an umbreon is to trade to an ruby/sapphire/emmerald version were time is used, (remeber that at the start mom ask you to set the clock in r/s/e and in kanto there's no clock)

Answer 2

Sorry, but I believe Firered and Leafgreen are only kanto, so you can't evolve into umbreon as it hasn't been introduced yet. You can do Flareon, Jolteon, or Vaporeon however :)