Can't upgrade

Every time I try to upgrade to WP 3.1.3, I get this message:

Downloading update from http://ww.wp.xz.cn/wordpress-3.1.3.zip

And nothing happens. I don’t know what version of WP I have, but it’s old. I have downloaded the Zip file to my computer, but I don’t know what to do with it. I wish WP were as user-friendly as Posterous!