What versions of Windows, iTunes, and iOS are involved? iOS 11 requires a minimum of iTunes 12.6.3.6, which in turn requires Windows 7 or better. If Apple Software Update isn't fetching a newer version when it ought to be able to then go to the source: iTunes - Download iTunes Now - Apple. Should you have difficulties upgrading see Troubleshooting issues with iTunes for Windows updates. Should iTunes not show your device after the upgrade see iOS device not showing in iTunes for Windows.
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