Chansey can only evolve into Blissey in generation II and later. If you're playing Pokémon Red, Blue, Yellow, or Green, you'll need to transfer to Gold, Silver, or Crystal in order to be able to evolve Chansey.

How do you evolve chansey?

Unlike most Pokemon, Chansey requires special conditions to be met before it will evolve. In order to evolve Chansey, you'll need to raise its happiness level up to 220. To do this, you'll simply give Chansey the Soothe Bell. This handy item will increase the holder's friendship and will make this process much quicker.

How rare is chansey in fire red?

Chansey can be found in the Safari Zone in the Center at level 23. It has a 1% chance of appearing in the center area and a it has a 4% chance of appearing in area 2... Good luck!