from reddit:
Go to control panel
-Go to Hardware and Sound; Devices and Printers; Device Manager; Portable Devices (look for "Apple iPhone")
-Right-click, then select properties on the menu list.
-Under the "Driver" tab menu, confirm the update driver button is not greyed out.
-If the update driver button is grey and unselectable, you need to select and click "Change Settings" button located near the lower left corner.
-Now you can click the "Driver" tab menu near the top, so you can see the "Update Driver" button.
-Select "Update Driver".
-Select and click "Browse my computer for driver software".
-For Windows 8.1 64 bit, it should be located in the folder: C:\Program Files\Common Files\Apple\Mobile Device Support\Drivers.
-Click Next, and let the driver re-install