changing DNA of an organism
Erm. Sort of. Kind of depends on your definition. It hasn't been done for multicellular organisms though.

Craig Venter et al did this exact experiment a few years ago with a completely synthetic Mycoplasma genome.

http://www.jcvi.org/cms/press/press-releases/full-text/article/first-minimal-synthetic-bacterial-cell-designed-and-constructed-by-scientists-at-venter-institute-an/

If you just want to change small segments of DNA that's where cloning and gene therapy come in, and we do that A LOT.