When an Eevee evolves into a leafeon and you dont like it can you have it evolve into a umbreon or can it only evolve once?

Unfortunately, once evolved eevee can't evolve so sorry but for now you're stuck with leafeon

not necessarily. if you have another eevee that is a different gender from your leafeon or you have a diddo, you can breed them in the Pokemon daycare center and try again.