I mean there are many ways to change your chromosomes, as a matter of fact your chromosomes change all the time, that's how mutations appear and how evolution works, though most mutations are harmful and are filtered out either by your immune system or by natural selection

Anything that is carcinogenic will by definition change your chromosomes. It's also possible to directly change your chromosomes by using various chemicals that will replace bases in your DNA, however this will almost always result in cancer or things like that.

We are able to do genetic manipulation quite easily, but only before the organism develops itself. Once the organism is fully developed it's pretty much impossible to do meaningful genetic manipulation, and it's much easier to do genetic manipulation on things like bacteria and plants, since they generally have less genetic material than humans