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I don't think we can -- at least not immediately. I think that human nature is something that is innate and instinctual; something that is built into us. I think that our nature predisposes us to have particular traits such as selfishness -- so that we do what is in our best interests. This makes sense because hundreds of thousands of years ago, the people that looked out for their own interests (even at the cost of others) were more likely to survive.  Thus, because of natural selection and 'survival of the fittest', those that survived were able to pass the trait of selfishness and self-preservation (along with other innate traits, such as disgust etc) 

However, I believe that human nature can be affected by how we 'nurture' it -- or the rules and standards society imposes on us. The expectations and morals that are imposed on us, such as what is 'right' behavior and what is 'wrong behavior, help to keep our instincts in check. For example, if you find $1,000 lying in the hallway of your dorm, your first instinct would probably be to pocket it. However, because of our morals and conscious, we don't and return it to the person it belongs to.