I kept seeing the instructions for how to backup your blog but when I went into my settings there was no button there.
(Note: This applies to desktop view, not mobile.)
I have more than one blog (they’re listed on the right-hand side of my settings page, under the “Account … Labs” menu). I had to click the name of the blog there. Then it takes me into the settings for that particular blog (this is the page that shows you a preview of your desktop header at the top).
At the bottom of that blog-specific settings page, there is a setting that says “Export”.