If you not started the project, this is the best possible way.

flutter create --org com.yourdomain appname

If you already created the project, install this package in your dev_dependencies and run this command.

flutter pub run change_app_package_name:main com.new.package.name

This will change all the complex stuffs of changing package name in android. And right click on the iOS folder of flutter and open with Xcode.

In Xcode > select Runner > In general tab you can change the bundle Identifier.

This is the easiest possible way if you already created a project and don't know the complexity of change the package name.

For more details-

https://stackoverflow.com/questions/51534616/how-to-change-package-name-in-flutter