Make us become less fertile as conditions change...except we already do that.
Slowing down the human reproduction rate overall will not work - The cause of the human reproductive boom is not our high reproductive rate (we have one of the lowest reproductive rates of any species in the world) but our reduced death rate. Population growth without limitation is exponential; no matter how slow this exponent is, at some point it will skyrocket. Like Thanos's stupid plan, reducing human reproductive rate is a temporary solution at best; at some point we will exceed the carrying capacity of the planet anyway...
Unless we are able to detect changes in our environment and slow down our own reproductive rate when our population outstrips the carrying capacity of our environment. Many animals do just this, especially large predators like bears (which is why pandas are having so much trouble reproducing in captivity). However, these animals tend to become less fertile as food becomes scarce. The problem with doing this to humans is that we are so good at keeping ourselves fed that many are unlikely to notice the change until it is already too late. What humans really need is to become less fertile as life becomes easier.
Which we already do.
Population growth in developed countries is leveling off and in some cases (like Japan and Germany) has already started to decline. Because of this, most modern scientists agree that overpopulation is not a danger to the planet. As long as countries continue to develop, eventually the global population should stabilize with an average of 2 children per couple. Population growth will stop.
The real problem is not the number of humans, but the behavior of humans. A global measurement of human population growth vs environmental impact gives a skewed picture, because the population growth and environmental impact are not happening in the same places. Developing countries have the highest population growth, but the smallest environmental impact. Developed countries have the smallest population growth, but the highest environmental impact. It is that latter that must be targeted to inhibit environmental damage, not the former.
Directly or indirectly, you will need to modify behaviors
In prehistoric times, there were periods of plenty and periods of famine. In order to survive, humans would stock up during the periods of plenty in order to survive the periods of famine. The problem is that even when in a comfortable environment, the human body wants to consume as much as possible in preparation for the lean times ahead. In modern times, however, those lean times never come (unless our unrestrained consumption causes them to come).
So how can we fix this without modifying psychology directly?
One of the biggest sources of environmental damage occurs due to heavy meat consumption. The human body craves high-energy food, and given the option (which we have now, while we didn't in the past) will eat as much of it as possible. Land clearing for livestock, emissions and runoff from farmland cause immense amounts of environmental damage.
We already limit our own reproduction when conditions are too easy. If there was a way of connecting the drive to eat meat to this same part of the brain, such that developed countries were less inclined to eat meat, this would benefit the environment greatly. This can likely be done without significantly altering overall human psychology.
There are other changes that can be made to make humans less inclined to consume under comfortable conditions - like reducing the need for competition - but these will likely require more invasive psychological modification.