It doesn't, not in leaf green atleast. If you trade it to a more recent game then evolve it by happiness.
Chansey > level up with max happiness > Blissey
Just travel with it and battle with it and level it up
u need to make chansey really really happy then level it up once then it evoles
Happiny will evolve only when it has max friendship with its trainer it can be level 100 when it evolves if you don't treat it good the earliest level you can get Chansey is level 2!
level 32
it will evovle at level forty-five
it evolves when having the maximum amount of happiness.
she can not
Level 33.
it evolves at level 40
level him up to level 30
At level 55