The world is most absolutely not "overpopulated" but, even if you don't agree, you still don't have to worry.
The rate of growth has stalled, meaning that our population keeps getting bigger, but at a slower rate.
The way things are going, we will topple off at almost 10 billion people, and then we will naturally slow down. Our numbers will keep falling until we reach a comfortable point.
This will happen naturally.
There are places that are "overpopulated" yes, but there are also many developed countries who are not sustaining their populations.
It is all about the relocation of people, somebody is gonna have to live in that abandoned (insert abandoned castle here).