Microsoft has released the final build of Service Pack 1 for Windows 7 and Windows Server 2008 R2. Windows 7 Service Pack 1 includes a number of hotfixes and security updates. Windows Server 2008 R2 Service Pack 1 includes a number of new features in addition to hotfix and security updates such as new Dynamic Memory and RemoteFX. For more details on what is new check out the official Microsoft Release Notes.
Service Pack 1 will be available over Windows Update but you can download the full installs from the Microsoft Download Center.
Before you install take a look at the Microsoft recommendations to ensure the update will be sucessfull.