I have a couple of articles for you that will help you troubleshoot this issue with updating your iOS devices on your computer. The first is about troubleshooting your USB connections, and can be found here:
The next step you may want to try would be reinstalling iTunes through the instructions found in the following article:
If you are still seeing issues after attempting these steps, you may find some information about the specific alerts you are seeing by referencing this article:
Thanks for using the Apple Support Communities!