A preview release of Windows 8.1 will ship on June 26, and Microsoft has given some details on how this will work. It will be distributed through Windows Update and when the final 8.1 release is available, that too will come out on Windows Update. If you install the preview, however, there won’t be any good way of updating to the final version. While installing the final release will preserve data and user accounts, all applications (both Metro and, on x86 Windows, desktop) will have to be reinstalled.
However, if you skip the preview, you’ll be able to install the 8.1 update as a straightforward upgrade. This won’t require reinstallation of any applications.
Installing the update will take about 4GB of disk space, but the installed size should be a little smaller than that of Windows 8 due to smarter use of compression and removal of temporary files.