How can I upgrade my iPhone iOS

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

The highest possible for that model is iOS 12.5.6.

Is your device compatible to update to the new version? See the article(s) below. 

iOS 16 compatible devices

IOS 15 and iPadOS 15 Compatible Devices.  *

iOS 14 and iPadOS 14 Compatible Devices.  *

iOS 13 and iPadOS are compatible with these devices - Apple Support.  *