[Theme:twentytwelve] Editing the theme (updates)

Hi,

So I recently moved over to WordPress for building client websites. I can code myself and pretty much understand all the workings of the themes but one thing I cannot understand is the updates.

I asked a question before about this for just css and so was told about child themes, which worked great but now I am wanting to make some changes:
move the search bar into the menu
backlink in the footer
page wider

And so I am thinking if I make all these changes it is going to overwrite it all once it wants to update?
How can I prevent that from happening?

Also on another note, as I am relatively new to this, does anybody know how I can style the search bar. I don’t mean background-colors as such as I can do this. I am talking about changing the big bulking search button to a little magnifying glass like page lines has.

Thanks and all the best

Dan