For upgrading from store.
Windows 8.1 - Upgrade To from Windows 8 or 8.1 Preview
Upgrading using the Windows 8.1 GA DVD OF YOUR VERSION (core or Pro) .
For upgrading from the desktop.
How to Activate retail Windows 8.1 with Windows 8 Product key.
A Windows 8 product Key can not be uses to Upgrade to Windows 8.1.
1) Use a Windows 8.1 product Key to Upgrade.
For Upgrading from Windows 8.1 Preview.
For clean install.
2) Activate with your Windows 8 product Key.
3) Open PC Setting.
4) Click on Change product Key.
5) Type in your Windows 8 product Key & Activate.
Product Key Number - Change in Windows 8
Product Key - Find for Windows 8
I use option five.
RWEverything | Read & Write Everything
NoteNote that although you may previously purchase a legit Windows 8 product key, the product key is unlikely able to be used to install or upgrade to Windows 8.1. Hence you will need the following product key to initiate the install or upgrade:
Windows 8 (core): 334NH-RXG76-64THK-C7CKG-D3VPT
Windows 8 Pro: XHQ8N-C3MCJ-RQXB6-WCHYG-C9WKB
Windows 8 Enterprise: MNDGV-M6PKV-DV4DR-CYY8X-2YRXH
KMS setup keys (8/8.1): Appendix A: KMS Client Setup Keys
The keys above will let you install 8.1, but that's all it does. After installation is complete, you will still have to activate with your own Windows 8 key.
Windows 8 and Windows 8.1 ISO - Download or Create
How to download Windows 8.1 ISO with a Windows 8 key.
How to download the Windows 8.1 ISO