iOS 7 I want iOS 8 latest version
Shut the iPhone completely off and turn it back on.

Make sure the iPhone is at 100% charge and still plugged into the charger.

Attempt to do an "over the air" update wirelessly via Software Update under Settings.

If that fails then update the iPhone via iTunes.

Have a look at Update the iOS software on your iPhone, iPad, and iPod touch for steps using either method.

Make sure you have adequate free space to preform the upgrade.  Have a look at If you need more space for an iOS update if need be.