Lots of questions at this site consider how humans could be enhanced with genetic engineering. But, if you are an alien, you may have the opposite priorities.
These aliens are civilized and have their own code of ecological ethics, so they don't want to exterminate humans (and would prefer to prevent massive, sudden population decline in humans, to the extent that this is possible).
But, they do want to prevent humans from continuing to develop into a species that is technologically superior to them that could be a threat to the aliens, should humans gain access to interstellar transportation technologies that the aliens already possess, either developed by humans on their own, or by humans reverse engineering it from alien technology in a manner that would require an ability on the part of the humans to understand the underlying physics with this huge hint available.
The aliens believe that humans are currently at least seventy years away from developing interstellar transportation technology on their own (assuming that they don't intervene).
(This interstellar transportation technology involves fake physics of the usual sort for science fiction, such as space folding in dimensions not known to current science, that is at least as difficult as general relativity and quantum physics to invent, but is easy enough for a trained person to operate. The viability in the real world of this fake physics technology is not, itself, part of the question).
The aliens are also aware that humans are at risk of self-extermination and would like to prevent that from happening. They also don't want to make a change so extreme that the affected people cannot properly be called humans, even though they are genetically modified humans.
The aliens also value elegance and minimalism. They want to make changes that are as subtle as possible to achieve their goal.
Also, the more subtle the effect, the more time it will take to detect it and mobilize a reaction to it, and the less time humans have to respond to the germ line retrovirus threat discussed below.
Precedents In Fiction
The closest analog to this that I can think of in fiction is Asimov's "brain fever" concept.
The Germ Line Virus Mechanism
The aliens have advanced bioengineering technologies and can engineer a germ line virus that would impact as many humans as viruses for the common cold or similar common viruses do today that would change the genetics of human sperm and egg cells causing everyone with at least one parent infected, before the sperm and/or egg cells that give rise to the child are formed, to have the genetic changes that the aliens desire.
This technique would not reach 100% of the human population (at least not immediately) but would reach a very large proportion of the human population and would disproportionately impact humans that are not in isolated (usually technologically backward) communities. It would probably impact at least 90%-95% of the non-isolated human population in each generation, more or less randomly. So, in the second generation, less than 1% of the human population would be uninfected, and assuming more or less random mating, in the third generation, the unaffected non-isolated human population in the third generation could be in the hundreds or less.
So, a solution that it likely to be possible to thwart if there are just a few hundred random people who are unaffected probably isn't sufficient.
What should the aliens target to modestly devolve and tame humanity in a way that prevents them from making technological progress (and that may even represent a step backward for mankind) without exterminating the species or causing a mass die off from the virus itself?