DNA of the human body?
The classic analogy here is that of a blueprint. For example this one I found here which is kind of relevant:

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If you use the blueprint above to make yourself a little sister and later change the blueprint, the sister you built will not be affected. 

That is only part of the story though, the analogy can only go so far. DNA is constantly "read", what this means is that genes are transcribed to RNA which in turn is translated into proteins. To use a horrible simplification, genes are the blueprints for proteins. Therefore, if you change the gene, and the cell then reads from it to create a protein, it will indeed create a different protein than what the original gene would have coded for.

The point I am trying to make is that depending on which genes are changed, you can either have no effect or a huge one. Some genes are only read once when the cells are reproducing, others are read constantly as the proteins they code for are used up and then produced anew. So, if the gene you change is one that will be read again during the cell's lifetime, then you will see a change. A classic example of this is cancer. If, on the other hand, it is a gene that will not be read again, then the changes will not have any effect.