Presumably sequence your genome early on in life while you're still healthy, then somehow (nanobots?) go in later in life cell by cell detecting things that have changed and fixing them.

That's my uneducated but logical sounding assumption.

A manipulated virus will do fine. In fact, they've been doing a trial on something like this for years now with some american heart patients, except the genome us not their own. Defeating genetic heart disease by hot-swapping the genome is somewhere between obvious and mad and also exciting.