Evolving a Cherubi is as easy as evolving most other Pokémon in Pokémon GO. All you’ll need is 50 Cherubi candy, which are given to you as you catch the Pokémon. Of course, this means you’ll need to catch multiple of them, though if you feed it a Pinap berry before capturing it, you’ll receive even more candy than before.

Unfortunately, Cherubi has a very low spawn rate in the game, so you’ll have to do some hunting to find it, or wait for a special event where the Pokémon’s spawn rate is higher.

Source: https://www.distractify.com/p/how-to-evolve-cherubi-pokemon-go