Actually, there's not much I'd want to say. I believe that even minor changes to the past could have drastic effects on the future. Everything I am and everything I have is the result of the choices I've made throughout my life.
There are a couple things I'd want to say to others, though.
I'd tell my grandfather to get that cough checked. It's not just a minor heart arrhythmia, it's pulmonary fibrosis and it's going to kill you.
I'd tell my grandmother to get her cholesterol checked. It's going to lead to a series of strokes that will leave you senile, infirm, and eventually kill you.