There is no way to uninstall the Windows 8.1 upgrade
. If you did not create a Windows 8 system image before you performed the update, you are left with the option of reinstalling Windows 8 from bootable media (sounds like you don't have this either,) or from the online installer.
Provided you have a valid product key, you can click the appropriate link on the following site and begin the process of reinstalling Windows 8:http://windows.microsoft.com/en-US/windows-8/upgrade-product-key-only
Make sure you have backed up all of the information you wish to guarantee safe keeping of before proceeding, as it can easily be lost during the installation of any version of Windows.