That’s an interesting question. We can’t really change the DNA sequence (the code, such as A,T,C,G) for the better – it only really gets worse by things like smoking and excessive amount of sun exposure/sunbed due to mutations they cause. An accumulation of mutations in important genes can lead to cancer.
But let’s think positive – is it possible for us to change our DNA for the better? Perhaps! There is something called “epigenetics” which are chemical changes to the DNA molecules. These changes turn some genes off and some genes on. It’s suggested that perhaps we CAN change the epigenetics of our body. Lifestyle (such as diet) and enviroment can change our epigenetics. We don’t quite understand what kind of changes/how these changes occur at this moment, but maybe we will know more in the future!