Question: how to evolve an eevee into umbreon at night?
Answer: Make the Eevee you want to evolve your Buddy. Walk with your Eevee Buddy for at least 10 KM and earn two at least Eevee candy. (You need to have earned two candy for this to work.) While that Eevee is still your Buddy, hit the Evolve button during the day to get Espeon, or at night to get Umbreon.