Question: how to evolve eevee to umbreon reddit?
Answer: Walk Eevee for 10 km (NOT during an event that affects buddy distance).
Get 2 candies from it (i.e. it doesn't work if you walk 4 km, then switch, then resume and walk 6 km). ... 
Evolve it during the (in-game) day/night while it's your buddy (switching out and back in between walking and evolving is allowed).