The simple answer is, yes (as has already been pointed out by others).
However, to expand upon what some people have said, the more complicated answer is that it depends upon what you mean by modify. As others have pointed out, if you're talking about humans, we don't currently have the technology to modify the DNA of all cells concomitantly. For that matter, we can't to my knowledge do it for any macroscopic multicellular organism. Nor do I suspect would we probably want to do that, not without a much better grasp on biology then we currently have.
However, there are numerous technologies available for modifying DNA in a more targeted fashion. Others have pointed out engineered nucleases, enzymes that cut DNA but whose target we can control, as key technology for genome engineering. They are especially useful in what is called gene targeting, or the modification of an endogenous gene (normally occurring gene in the organism) through homologous recombination (others have provided decent references).
There is another standard method of gene-therapy/genome engineering which hasn't been touched on as much, which is the use of retroviruses to permanently modify a host’s genome. Basically because retroviruses as a part of their live-cycle naturally use their own genetic material, in this case RNA, to modify whatever it is they infect by converting it into double stranded DNA and stably integrating into the host genome, we have used them to integrate whatever sequence we're interested in. Of course, it should be noted that this method is mostly a delivery vector, so it could be used in conjunction with engineered nuclease methods, but you can also just insert a gene of some sort.