Theme-child

Hi,

I have a problem with theme-child, i make child theme with plugin Child Themify, after i already make changes to style.css. I activated child theme and did everything else in cusomize(photos and everything else) and everything was like i wanted. But after update i lost everything in style.css, footer.php etc. i have to do all over again.

Can someone help me how to do it right so next update doesn’t do this again?