Can't update SQL Server 2008 to SP3, it only offers the option to create a new instance

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http://serverfault.com – A colleague is attempting to update our server's SQL Server 2008 version 10.0.1600.22 (not R2) install up to SP3. Unfortunately, we can't work out where the "Update" button is; the installer seems to be set on creating a new instance, which isn't what we're after. Windows Update doesn't seem to be finding the update either, so we can't do it that way. What are we missing, here? (HowTos)