For those of you who are on the Windows 8.1 Preview, upgrading to the final release of Windows 8.1 is possible, but you will need to re-install all your apps. Unfortunately, you will only be able to transfer your data from the Preview to the final version of Windows 8.1 and all desktop and Windows Store apps will have to be re-installed.
However, for those of you who stuck with Windows RT or Windows 8 and avoided the Windows 8.1 Preview, you will be able to update to the Windows 8.1 update without the loss of anything.