You can use the OS X combo update to bring your system up to date, and in the process, replace most of the key system software files with the most current versions included in the updater.
Unlike the incremental approach used in the Software Update system, the combo update does a wholesale update of all affected system files.
So Apple has recently started automatically downloading the latest version of Mac OS onto most people's computers.
With the latest version of Mac OS, Sierra, Apple is automatically downloading the major update, which takes up about 5GB of space.
Click the link for the version you're interested in, then set the view option to Alphabetical, and scan the listing for the combo update you need.
All of the combo updates will have the word "combo" in their names.
Likewise, the Mac OS X v10.5.8 Update (Combo) requires that OS X 10.5.0 or later be installed.
Apple keeps all of the OS X combo updates available on the Apple support site.
A quick way to locate the right combo update is to stop by the OS X Support Download site.
There you will see the three most recent versions of OS X, along with a link to older versions.
Double-click the installation package to start the install process, and follow the on-screen prompts.