How can I regenerate ios folder in React Native project?

The process you need to follow is so similar to renaming a react native app. Basically you just need to run react-native upgrade in your root project directory. For further info you can check another question here. The instructions below explains how to create another react native project based on a copied one with a new name.

• First copy the directory which your to-be-name-changed application exists. And go to your newly cloned directory.
• Change the name at index.ios/android.js file which is given as a parameter to AppRegistry.
• Change the name and version accordingly on package.json
• Delete /ios and /android folders which are remaining from your older app.
• Run $react-native upgrade to generate /ios and /android folders again.
• Run $react-native link for any native dependency.
• Finally run $react-native run-ios or anything you want.

Simply remove/delete android and ios (keep backup android and ios folder) and run following command:

  react-native eject

Supported version : 
    react-native <= 0.59.10
    react-native-cli <= 1.3.0 



   react-native upgrade --legacy true

Supported version : 
        react-native >= 0.60.0
        react-native-cli >= 2.1.0 

Ref : link

Edited(March 2021)

npm i react-native-eject
npm i @react-native-community/cli
react-native eject

will generate the both android and ios folder.

Ref.: link