I need to update the iOS on my iPhone 6

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 can be updated to iOS 12.5.6, no further.

The iPhone 6 will not support an iOS higher than 12.5.6. If that is what you have, you will not be able to update.

iOS 12.5.6 is as “far” as your older device can be upgraded.