There are a number of possible approaches to this. I will discuss four of them.
First, we could try governmental policies to limit family size. China has done this through its one child policy. Other less authoritarian countries could do it through things like increasing taxes on people with more than a given number of children.
Second, we could try to promote the knowledge and use of contraceptives. We could have governmental and nongovernmental programs that would disseminate the knowledge of contraceptive use, particularly in developing countries. We could also make contraceptives more cheaply and widely available.
Third, we could try to raise people’s incomes. One thing that has proven true in a variety of countries is that people tend to have fewer children as they become richer. It appears that children are less economically valuable to people in rich world countries. The more that we can promote economic development in developing countries, the more we would be likely to drive down the birth rate.
Finally, we might consider that there is not a problem of overpopulation. Instead, the problem is overuse of resources. If we could give people a good standard of living without using as many resources, we would greatly reduce the problems that are associated with overpopulation. This would mainly entail developing more technologies that use less energy.
All of these would be potential ways to reduce overpopulation or its related problems.