How to get Umbreon in Pokemon GO is a fairly simple process.
Niantic Labs knew they would need to divine a unique system of evolution when implementing Eevee into their AR mobile title, Pokemon GO. Unlike mainstream games, trainers don't have the luxury of evolutionary stones nor visiting special secluded evolutionary locations. Fortunately, they quickly developed a solution to keep the experience authentic and specific to each Eevee-lution.
Here's how to prompt that system to give you Umbreon.
How to Get Umbreon in Pokemon GO
The first thing you'll need is an Eevee. Any Eevee will do, so long as you approve of its stats and base CP. Eevees can be found just about anywhere on the map. They're a fairly common spawn in semi-wooded areas and can hatch from 5km eggs.
Once you have such an Eevee, you'll need to make it your buddy and go on a series of long walks. The goal is to hit 10km while this Eevee is with you as your buddy. This is meant to simulate the level of "friendship" needed in previous games for Umbreon and Espeon.
After accruing enough friendship, check on your Eevee's profile during the night. You should notice that its "Evolve" button depicts a silhouette of Umbreon rather than a question mark. If this isn't the case, continue walking, feeding, and interacting with your buddy.
You should only choose to evolve during the night if you want an Umbreon.
Additionally, if you don't want to wait and go about the process the long way, you can use the nicknaming trick. The appropriate nickname for Umbreon is "Tamao." Simply name your Eevee this and evolve it to receive an Umbreon. Be warned, however, that this will only work once.