Q: Fail to install Apple Software Update 2.2 (has an invalid signature).

Run Apple Software update as administrator - this fixed it for me

Phil

Same issue for me.  Running as an administrator did not help.  It would be nice if Apple would make this available as a standalone download.

This is how I fixed it.

I went to Tools\Open Downloaded Updates Folder...  I left the window open.

Clicked Install and when it started verifying the warning message appears, but instead of clicking ok I waited and dragged the installer to my desktop before Apple Software Update deleted the file. Because it wont delete the file until you click ok on the message.

I closed Apple Software Update and ran the installer from my desktop.

-David

Hi dear friend

open the "Apple Software Update"  and Choose from the "Tools" select "Download Only" And then manually install the file

See Troubleshooting issues with iTunes for Windows updates if you have any problems updating iTunes. Note that in some cases Apple Software Update may have stopped providing you the latest builds. At the time of posting the current builds of iTunes are 12.3.3.17 for Windows 7 or later and 12.1.3.6 for Windows XP & Vista.

tt2

Thank You.  Your workaround worked for me!!