Where can you change Shaymins forme?

Question

Its all in the tiitle

Answer 1

You need a gracidia flower to change shaymin.

 There should be a lady in the Pokemon center in Lacunosa Town who talks about the Gracienda flower. I'm sure that if you have Shaymin with you, she will give you the Gracienda flower. You might have to do this during the time that the Gracienda works, and Shaymin might have to be first in the party.

PS it needs to be the event shaymin with the fateful enounter in it's summary

Answer 2

Use a Gracidea Flower on it during the day.
>While it is not known to evolve into or from another Pokémon, it has a second Forme activated by using a Gracidea flower in the daytime. Its original Forme, Land Forme, will then become Sky Forme. -Bulbapedia

The Gracidea Flower can be found in the Pokemon Center in Lacunosa Town.

Answer 3

You don't change it in a place. If you have an event Shaymin, take it to Lacunosa Town's Pokemon Center. Then, talk to the lady inside with Shaymin in front of your party. Then, she'll give you a Gracidea for Shaymin. Use it on Shaymin to change it into Sky Forme. You can only change it when it is day and Shaymin is not frozen.