To update to the preview you're going to first have to grab an optional update. I think this is a crude entry barrier so Joe Average doesn't upgrade by accident. You won't see it in the metro style UI you'll have to look for it through the desktop WU panel.
Then you go through the Windows Store to download and upgrade to 8.1 Preview. You'll need about 4GB of free disk space to complete the upgrade (it may actually be lower as they are/were still working on reducing this)
Upgrading from 8.1 Preview to 8.1 RTM will retain your user accounts and data, but blow away everything else. I don't know the details but it sounds like the old clean-install and migration rather than an actual in-place upgrade.
If you're worried about disk space with installation files lying around, you also have the option of using the Reset/Refresh function which will roll you back to Windows 8. From there you can go to the store and upgrade to 8.1 RTM. But the truth is I've no idea how they handle the clean up process because I haven't done it. However, they have said the install footprint of 8.1 is smaller than the footprint of 8.0, so I'm guessing they won't want to undo all that good work with a sloppy update process.
What I would suggest is that if you're not sure then just don't upgrade to preview, and certainly not on a production machine or one you can't do without. Secondly, the fact that you can upgrade/migrate from 8.1 Preview to 8.1 RTM and keep your data is almost arbitrary, because I would want to back up that data anyway.
If you have a x86/64 Windows 8 box it may be worth digging out your DVD so you know you've got a roll back option. If you've got a Surface I would strongly encourage you to create a USB recovery key just in case.
It's beta software after all.
Then you go through the Windows Store to download and upgrade to 8.1 Preview. You'll need about 4GB of free disk space to complete the upgrade (it may actually be lower as they are/were still working on reducing this)
Upgrading from 8.1 Preview to 8.1 RTM will retain your user accounts and data, but blow away everything else. I don't know the details but it sounds like the old clean-install and migration rather than an actual in-place upgrade.
If you're worried about disk space with installation files lying around, you also have the option of using the Reset/Refresh function which will roll you back to Windows 8. From there you can go to the store and upgrade to 8.1 RTM. But the truth is I've no idea how they handle the clean up process because I haven't done it. However, they have said the install footprint of 8.1 is smaller than the footprint of 8.0, so I'm guessing they won't want to undo all that good work with a sloppy update process.
What I would suggest is that if you're not sure then just don't upgrade to preview, and certainly not on a production machine or one you can't do without. Secondly, the fact that you can upgrade/migrate from 8.1 Preview to 8.1 RTM and keep your data is almost arbitrary, because I would want to back up that data anyway.
If you have a x86/64 Windows 8 box it may be worth digging out your DVD so you know you've got a roll back option. If you've got a Surface I would strongly encourage you to create a USB recovery key just in case.
It's beta software after all.