There are many forms of Eevee evolutions, and Umbreon is one of the more popular variations. Since all Eeveelutions are available for capture or evolve Pokémon Scarlet & Violet. Here's a guide on how to get Umbreon in Pokémon Scarlet & Violet.
How to evolve Eevee into Umbreon in Pokémon Scarlet & Violet
There are two ways to find Umbreon in Pokémon Scarlet & Violet. The first is to find it has already evolved in the wild, and the second is by building a high friendship with an Eevee and evolving it at nighttime.
The first method is a lot easier, as we found our Umbreon in the caves between Medali and Tagree Thicket, to the north of the Great Crater, beneath Glaseado Mountain. In the middle of the two caves is a convenient Pokémon Center. Even though the Pokédex entry says it's a rare spawn, we found a few quite often; Glimmet was far harder to make spawn.
The second method takes more effort but feels more rewarding as you need to do some leg work. You need to have a high friendship with Eevee and ensure Eevee does not know any fairy moves upon evolving, or it will turn into Sylveon.
How to raise friendship quickly in Pokémon Scarlet & Violet
There are two main ways to raise friendship quickly in Pokémon Scarlet & Violet. The best way is by going on Picnics, feeding it sandwiches, playing with it, and bathing Eevee. Do this multiple times, and your friendship with Eevee with rise.
The next way is by using Berries to boost the friendship between the trainer and Pokémon, however, this will decrease a stat. So if you want to use Eevee in a team, then the Berry route is not the best method. But alas, it may be the faster route. Weigh up your choices.
How to get an Umbreon evolution from an Eevee in Pokémon Scarlet & Violet
Once you have enough friendship with Eevee, make sure no Fairy moves are in Eevee's arsenal. Fight with Eevee until they are about to level up, or use EXP candies. Wait until it is nighttime and then save the game. Level up your Eevee while it's nighttime, and if its friendship is high enough, it'll evolve into Umbreon.
If the evolution doesn't trigger. Reload the game, go on more Picnics to eat more Berries, then try it once more.