To evolve Eevee into Umbreon in Pokemon Go, you can use the name trick by renaming your Eevee "Tamao" before evolving it. However, the name trick only works once per evolution type, so if you have already used it to get an Umbreon, you will need to evolve Eevee into Umbreon the old-fashioned way by completing certain goals. One way to do this is to make Eevee your buddy and walk 10km with it while collecting two candies along the way. After that, you can evolve it into Umbreon at night while it is still your buddy.