How to move navigation bar below header image on Twenty Twelve theme

I must be a complete WordPress idiot – because I want to move my navigation bar below my header image but don’t understand how to do what the provided instructions is telling me to do – creating a child theme and such. It references css and ftp – things that I can’t grasp alone.

I’m only slightly comfortable making small changes to html code but am terrified I’m gonna ruin everything I’ve done so far.

Your help is sooo appreciated!