To install Windows 7 SP1 via Windows Update, simply open Control Panel Windows Update and click on the link “Important updates” On this page, make sure that Windows 7 Service Pack 1 is selected and click ok.
Now click the Install updates button and it will take care of the rest of the procedure, it will download the required components of Windows 7 SP1 and will automatically install and re-start your Windows 7 PC.
After successful validation, a Microsoft Windows Download Key will be stored on your system for future use.
Following the validation process, you'll be taken back to the initial download details page to obtain your requested download The validation process also determines if you have activated your copy of Windows.
This is the easiest and fastest way of installing Windows SP1.