Once you've update your iPhone, can you go back?

I've got the iPhone 4s and I've just updated the software to 6.0 but I'm not very keen on it. I was wondering if it was possible to go back to the original software that came with my phone without losing anything (contacts, apps, music, photos etc.)

Yes you can. Just google for the exact version of iOS you're looking for (for ex: iOS 5.1.1) and download the file. After that open iTunes where you can see the option to restore the firmware to retain the old settings, click the restore icon using the shift key and that will allow you to browse for the desired file. Select the downloaded file and you should be good.