How to upgrade existing GPT API credits from 3.5 to 4? | Platform Open AI

Hi All, How do we upgrade the existing API credits on from GPT 3.5 to 4?

By default, it is connected to GPT 3.5. Any solution to this?

On, GPT-4 should be available by default.

Also, the credits on are unrelated to the usage fees for ChatGPT and are individual.