Now you can download Windows 7 SP1 via Microsoft Download Center or Windows Update.Most people will get Windows 7 SP1 via Windows Update when it becomes available to them.And best way to do that instead of downloading the standalone installer you can use Windows Update option.Windows Update is the best installation method.
Before Installing Windows 7 SP1 you must backup your pc to DVD/CD,external hard drive or some other ways.And you should also check your PC for any known malware and viruses before installing Windows 7 SP1.
You can also check your PC using Microsoft free security software like Microsoft Security Essentials.
Check here How to install Windows 7 Service Pack1