You will need to purchase a license key for 8 Pro - you cannot upgrade a system that shipped with 8 to 8 Pro without buying a new license key.
Phone Dell Technical Support and request a Windows 8 or 8.1 Reinstallation DVD/USB.
You currently have the wrong version of Windows installed without a way of activating it (besides purchase of a new product key).
P.S. where did you obtain the Windows 8 Professional media? Do you have this from an unused license?
windows 8 after change in hardisk it come up with windows 8 pro.
What makes you think you can activate Windows 8 Pro on a system that came originally with Windows 8?
Please be honest with what you did. Windows 8 Pro does not "magically" install itself. YOU installed it.