Altering DNA is quite easy; all you have to do is spill certain chemicals on it or spend too long in the sun. However, to do it safely and controllably is impossible.
Wow great idea I am going to bathe in a bath of toxic waste and metalayted sprits then sit in the sun for hours and I'll be superman awesome dude.
No, the DNA is fragile and will get destroyed easy. it also must enter your cell to change you. if that does happen then that one cell will undergo apoptosis and kill itself to to reproduce altered...