Theme Update

Hello,

updating the theme overwrites even the png files i had replace like the logo, also it delete all files in the directory like logo.png.original,
also if i set non writeble permission the them wont update.
Question: why don’t you update just the needed files?
Question: A tutorial to make a child theme for the default DOWNLOAD THEME, i have modified some .php files and i wont re edit after every update.