iPhone screen freezes.. How to solve it ?

Some iPhone 6 and 6 Plus users have found that their device’s display becomes unresponsive when using apps. The problem has been reported on phones running iOS 9.3.1 and iOS 9.3.2, and has been known to occur multiple times a single day.

++Workarounds++

Use the lock button to turn the screen off and on again.
Hold down the Sleep/Wake and Home buttons until you see the Apple logo appear to reset your iPhone.
Factory reset and restore your device as a new phone, or restore it from a backup.

++Potential Solutions++

If none of the aforementioned workarounds fixes the problem, consider contacting Apple or going to an Apple Store. The problem may be with the hardware, not the software.