When you connect an iPhone or iPad to a computer, it shows a pop up window asking permission whether or not you want to trust the connected computer to access your data whenever your device connects to it. The window shows two options: “Trust” and “Don’t Trust”.
If you want to allow the connected computer to access your data whenever you connect to it, the answer is to press the “Trust” button. So in the future, it will not ask for permission every time you connect your device to this computer. If you accidently select the “Don’t Trust” option, it still gives you the choice next time you connect to the computer and then you can select the “Trust” option. There is however this problem with selecting the “Trust” button. Undoing the “Trust” option is a bit problematic. If you want to “Untrust” the computer from iPhone or iPad after you have “Trusted” it once, here is how you can do it:
Steps to “Untrust” Computer from iPhone/iPad
Open the Settings app on iPhone, iPad or any Apple device
Go to “General”
Now go to “Reset”
Select “Reset Location and Privacy”
It will ask for the device pass code; enter it and confirm that you want to reset the location and privacy settings of the device. (Performing this action will let the device forget all the trusted computers that you had previously selected and will ask your permission on all the computers in future. Also, it will reset your privacy and location settings which you might have customized earlier.)
If you want to configure the Location and Privacy settings, you can do it now (or later).
Exit settings when the action completes
Now simply re-plug the device to a previously trusted computer. Upon connection you’ll be asked afresh to trust the device or not. Select the “Don’t Trust” option this time and this computer will be “Untrusted” from your iPhone or iPad.
This is how to UnTrust Computer from iPhone and iPad. Fixing this problem was not so easy in versions before iOS 8, where one had to reset the device to factory settings or turn some options in iTunes settings in the computer. Currently iOS 8 and iOS 9 however, offer this rather easy solution to Untrust the computer from iPhone or iPad.