http://windows.microsoft.com/en-US/internet-explorer/download/ie-9/worldwide
To Uninstall IE9 Beta
1. Enter “appwiz.cpl” to open the Programs and Features panel
2. Click “View Installed updates”
3. Search for Internet Explorer 9, and then right click to select Uninstall
After the Uninstall process, you need to restart your computer and IE8 will be the Internet Explorer again.