How to change the key in Windows 8

When I upgraded from windows 7 ultimate to windows 8, it was all smooth and the there was no problem. When I clicked on update now, the message was displayed that "Activate windows before updating"
However all trouble started when I wanted to activate the Windows 8 by entering the KEY.
Clicking on activate link said Windows cannot be activate now, contact your administrator!
For complete 72 hour I kept on repeating the process of installing and uninstalling thinking that there would have been some problem in installation.
Then  I  came across this process as given below

To change the product key without first activating Windows, use one of the following methods:

Method 1

1. Press Windows+r key.
2. Type Command in run



Type slui 3 at command prompt

 3. Then press enter. you will be taken to Screen where you enter the Activation key.

4. This is the method I followed to activation.

Method 2

1. Swipe in from the right edge of the screen, and then tap Search. Or, if you are using a mouse, point to the lower-right corner of the screen, and then click Search.
2. In the search box, type Slui.exe 0x3.
3. Tap or click the Slui.exe 0x3 icon.
4. Type your product key in the Windows Activation window, and then click Activate.

Method 3

Run the following command at an elevated command prompt:

Cscript.exe %windir%\system32\slmgr.vbs /ipk <Your product key>

 You can also use the Volume Activation Management Tool (VAMT) 3.0 to change the product key remotely, or if you want to change the product key on multiple computers.