Either run the Java backend by using your IDE or by typing
mvn exec:exec
in the back-end folder. This will start the Java server at http://localhost:8080.
In the front-end folder, run
npm install
npm start
This will start the front-end server at http://localhost:3000. You can update the front-end code as the server is running in the development mode (i.e., npm start). It will automatically recompile and reload.