Simply rename your Eevee to get the desired evolution. To rename your Pokémon click on the desired Pokémon (in this case Eevee) and select the pencil next to the name type in the desired name and select ok. Then evolve your Eevee for desired Eeveelution! Make sure to restart your application to ensure that the changes have updated to the server. Renaming to Pyro yields you a Flareon. Renaming to Rainer yields a Vaporeon and renaming to Sparky gets you a Jolteon.
Pyro=Flareon Rainer=Vaporeon Sparky=Jolteon