We see a lot of movies and books with humans(or near human creatures) that live for hundreds of years. I've seen numbers for long lasting human ranged from 150 all the way up to 7000( and I'm sure that number is higher). Lets say I want to make an average human live for 200 years. What changes need to be met to allow this to work.
According to Richard Dawkins, there's nothing inherent in the human body that causes senescence. It's just that there has never been any evolutionary pressure towards longer living humans.
So the "fix" would be to forbid anyone from reproduce until they reached the age of 30 years or so. As the species began evolving from this artificial selection pressure, you'd gradually increase the minimum required breeding age.
This would not be difficult to accomplish from a technological perspective. However, it would be devilishly difficult to implement from a sociological/political perspective.
I couldn't find the direct reference but I did find this very similar question and answer on the biology stack exchange
I think this would probably work. The grandmother effect, which is one of the main theories for human longevity after fertility might indicate that human lifespan would increase if the children come later.
The Belyaev experiments which produced domesticated foxes by strong selective breeding took some 30ish generations to complete though they saw a significant effect in just 10. It could take a similar numbers of selection rounds in this cse. 4-500 years. Ethical issues aside, this is why most geneticists study flies.
Just have to disclaim here: We will never do this. Many unthinkable consequences would result.