Apple routinely releases updates to OS X that are available through the Software Update process or the Mac App store, depending on the version of OS X you are using.These software updates, available from the Apple menu, usually provide the simplest method for ensuring your Mac's operating system is kept up to date.Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
As shown in picture, app store tell me to update i Tunes 12.5.1, but in updates installed list, you can see I have already installed i Tunes 12.5.1. After that I was able to download i Tune 12.5.1 properly.
But actually, my i Tunes version is still 12.4.3 Now, I click the update button of i Tunes, then after download completed, nothing will install, and return this page to wait update. If that doesn't work, you can always download it directly from: Thank you for your interest in this question.
If you don't see the word combo, it's not the full installer.
The combo updates are stored as (disk image) files that will mount on your Mac as if they were removable media, such as a CD or DVD.
The downside to the combo updates is that they're huge.
The current (as of this writing) Mac OS X 10.11.3 Combo Update is just shy of 1.5 GB in size.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.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.The combo updates only update OS X system files; they don't overwrite any user data.