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Giving it a Soothe Bell, leveling it up, giving it vitamins, and even just walking around can get its Friendship level up enough to evolve to Umbreon. Just be aware that it will evolve into Espeon during the daytime, so if it starts to evolve and it isn't nighttime cancel the evolution via pressing B.
Also, if you have any befriending O powers yet, those could help speed up the process as well. Use them on yourself, then level up your Eevee. Avoid playing with it in Amie so you don't get a Sylveon instead.