Is it possible to change a gene in an adult body? Like eye color for example?
I know that in embryos you can easily do it because the cells have not multiplied as much, but wouldn’t an adult just be the same process but on a larger scale?
I think that they use programmed viruses to do the job, but I don’t know the current limits of science in this area.
It’s not being done outside of science fiction right now, and won’t be in the near future. “A larger scale” is an understatement - the number of cells that would have to be changed would be extremely high. And even when dealing with embryos with current technology, the success rate for actually changing genetic information is far below 100%.
On top of which, genes code for proteins, which then have some effect, like making an eye color. So if you change the DNA, the cell’s old proteins would have to be destroyed and replaced by the proteins coded for by the new DNA, then the new proteins would have to catalyze the new chemical reactions, etc, etc. How long that would take would vary significantly depending on the trait. I don’t know a lot about the molecular mechanisms of eye color determination, but I’d suspect even if you changed the DNA, it’d be tough to effect a rapid change in color. But I could be wrong.
Some current strategies involve the use of Adenovirus, or a safer version called Adeno-Associated Virus. These viruses are big and can hold large human genes. These viruses are being studied extensively and are even having their structures solved since knowing details about how the virus binds to hosts and knowing the structure of the receptor will allow us to modify it to attach to specific cells (like the eyes as you mentioned in the OP).
Gene Therapy is of interest in cases like Cystic Fibrosis or Diabetes where a gene insertion or gene correction could fix the problem. There have been tests on humans already, and there was a death several years ago using Adenovirus. IIRC they gave the subject something like 10^14 dose of adenovirus, which is a ton. Here are dozens of articles via Google on this incident.