You can clear the cache in a number of preinstalled Apple apps, including App Store, iTunes, iBooks, App Store and the Apple Watch app.
Launch any of the above apps, then tap on any of the icons at the bottom of the screen 10 times in a row.
For example, launch the App Store app on your iPhone – then hit the “Featured” tab in the bottom left-hand corner ten times in a row.
Your iPhone display will then go blank as the smartphone clears the cache.
Use this when you have problems with these apps.