Go to the CURRENT trunk of the entire WordPress file:
Download the zip archive at the very bottom of the page.
Extract the entire file somewhere on you computer.
Delete the zip file.
Open the extracted file and open WP Content, and then Themes.
Right click the TwentyTwelve file and zip it into a separate file.
Upload the zipped TwentyTwelve file through your website's Admin Panel.
Activate the Theme.
Delete the entire downloaded WP file from your computer. The current WordPress version 3.5 is only 1/2 completed.
I don't know of any way to update this "version" of Twenty Twelve, UNLESS you install the nightly build version on your server. If you do that, I believe that Twenty Twelve is now included in it. I tried that but my navigation menus wouldn't display, so I went back to WordPress 3.4.2. I'm happy with that.
esmi has been correct in noting that Twenty Twelve is still in beta as far as WordPress.org usage is concerned.