Steps to switch to React 18

To switch a project to React 18, there are generally three changes that need to be made.

Update the resolutions in your root package.json to the new versions of @types/react and @types/react-dom:

package.json

"resolutions": {
    "@types/react": "^18",
    "@types/react-dom": "^18",
},

Update the react and react-dom dependencies in your packages/app/package.json to the new versions:

packages/app/package.json

"dependencies": {
    ...
    "react": "^18.0.2",
    "react-dom": "^18.0.2",
    ...
},

Update packages/app/src/index.tsx to use the new react-dom/client API to render the app:

packages/app/src/index.tsx

...
import ReactDOM from 'react-dom/client';
...

ReactDOM.createRoot(document.getElementById('root')!).render(<App />);

Once these steps are done you should be able to run your app and see it working as before, except now using React 18.