To download the upgrade assistant, go to the Windows 8 download site, then click on the blue Download Windows 8 Release Preview button. The file is only 5MB, so it shouldn’t take long for the download to complete.
To run the upgrade assistant, double-click on Windows8-ReleasePreview-UpgradeAssistant.exe. It will immediately begin checking your PC for compatible apps and devices. Once it’s finished, the upgrade assistant will display how many of your apps and devices are compatible, and how many require your attention.