The Windows 8.1 Consumer Preview for various languages can be downloaded in .iso format from this page here.

As this is a time-limited beta evaluation. Ensure that you have the ability to reinstall your original to reinstall your original Operating System before beginning the installation of Windows 8.1. i.e. you have created a Recovery USB or have called Dell Technical Support and attempt to procure a Windows Reinstallation DVD.

Follow Windows Reinstallation Guide and Related Wikies for correct setup of Windows 8.1 and your original version of Windows. See the notes on the installation media.

Feel free to let us know what you think of the beta  on this thread and let us know if you run in any to any problems.

Ensure that you create recovery media (USB or DVD) from your earlier Windows 8 installation before installing the Windows 8.1 Consumer Preview.

Microsoft have not provided public .isos for Windows 8, they are only available if you purchase another license and have an earlier version of Windows Installed (i.e. not Windows 8 or later). 

The public .isos are only available for the Windows 8.1 Consumer Preview. Thus you may be unable to revert to your original Operating System if you don't have the recovery media. You may or may not be provided with media from Microsoft or Dell to install the final version of Windows 8.1 when it is released.