So a new Service Pack(SP) comes out and you're interested.
First, check the list of fixed SPR's from the Customer Portal. Make sure there are fixes in there that will make it worth your time to update. 😉
Second, download the update or full DVD with the installation manager.
Third, after the download completes run the update.
Uh, but where is the download exactly? Well in Windows 7 x64 it would be C:\users\<your login>\documents\SOLIDWORKS Downloads – Unless you put it somewhere else. If so, well, who knows, start searching for sldIM.exe this might refresh your memory.