There is no need to explain what overpopulation is as well as why it is so important to be able to find a reasonable solution.
Perhaps, the most obvious solution is to provide better education to people on the planet as a lot of people are simply unaware of various contraception methods. Thus, they do not even know that there are lots of ways in which it is possible to prevent pregnancy.
In addition, it is also important to create equal educational, work, medical and entertainment opportunities in all parts of the planets. In such a way, people will not have the need to travel to big cities or richer countries in search of a better life. What it ll boils down is that the concentration of people in one city will not be so tight.
What is important to highlight is that the issue of overpopulation can still be solved. The only thing which needs to be done is finding the right approach and doing everything one can to implement that idea in life as fast as possible.