Are you able to change evolvetions if Eevee already evolved?

As for any and every Pokemon that's evolved they cannot return back.

So if you evolved eevee into Jolteon and wanted umbreon, simply

breed Jolteon with ditto, a newborn eevee will hatch.

To make it happy, max it out on power ups,

HP UP, Protein, etc.

They can take only 10 of each, but this makes any Pokemon happy.