if i want to change my computer from Trust to don't Trust to iPhone, how could i do ?

I don't think there is a way to "un-Trust" it. However, you can:

Go into iTunes Preferences>Devices, and make sure the box Prevent...from syncing automatically is checked

Hook up your device to the computer, and select the device

Go to each profile page and uncheck the sync option on each

I sync my devices with my husband's Macbook Pro, but also hook them up to my Macbook Air all the time to check things for answers I'm providing on this forum. I have all of the Profile boxes unchecked, and nothing syncs on this Mac.

Cheers,

GB

It may be possible that if you go to Settings>General>Reset>Reset all Settings it will remove the trust, or at least cause it to ask again.

I tried your suggestion but it didn't work for me. My device didn't ask again.

iPad mini retina iOS 8.3

If that does not work, I do know that a restore would do it. If you restore your device, it will ask, because that clears the device. However, it will also delete all data from the phone. You would have to sync data back. I'm not sure if restore from backup will do it, but I do know that when I installed the update to iOS 8.3, it asked again.

on the computer you can untrust all devices and the you will be reprompted to trust/not trust each one again when you next connect them. - see iTunes: How to reset the Lockdown folder - Apple Support

I was not aware of that, thank you for the hint.