i want to update my phone

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. 

Note: the iPhone 6 Plus can be updated to iOS 12.5.6.

what is your current iOS version and what version are you trying to update to?

if your details are correct and you have an iPhone 6 Plus, you cannot update beyond iOS 12