How to Reset the “Trust This Computer” Alert and UnTrust All Computers from Your iPhone/iPad/iPod Touch
This resets all trusted computers from any iPhone, iPad, or iPod touch running iOS 8 or later:
1. Open the Settings app on the iPhone, iPad, or iPod touch
2. Go to “General” then to “Reset”
3. Tap on “Reset Location & Privacy”, enter the devices passcode, and confirm that you wish to reset all location and privacy settings on the iOS device
4. Exit out of Settings when complete (perhaps after you reconfigure the location and privacy customizations you had prior to the reset)
Now if you want to trigger the “Trust This Computer” alert on the iOS device again with a computer, simply plug it back into a Mac or Windows PC with a USB connection and you’ll see the familiar alert screen popping up again, allowing you to select either choice.