I have the flower but dont know how to use it on Shaymin.
To change Shaymin's form you need the Gracidea Flower. The Gracidea Flower can be obtained from a girl in Snowbelle City Pokémon Center if you talk to her with a Shaymin in your party.
Note: It can only be used between 5am and 8pm in the day and only on Shaymin that are not frozen.
For more help getting the Gracidea Flower check out this video here.
Hope I helped :)
You go into your bag, select the flower and it should have a "use" option, and just select Shaymin.
It doesn't work at night, if Shaymin is frozen or sometimes if the Shaymin is hacked.