You can't change the user agent string in the default browser. That's embedded in the closed binary and most who have tried to hex edit it have corrupted the browser.

A better option is to install Opera and/or Firefox. Both of which let you set the user agent string to a desktop browser. Not to mention both of these browsers let you view a web page full screen, while the default browser has no way to hide the address bar.

Better yet, Firefox for the N9 supports Adobe Flash which will be welcome if you're coming from an N900 and used to a better standard of browsing.