How to update my phone to a new version
How to update my iOS version
iPhone 6, iOS 12
An iPhone 6/6 plus can't be updated beyond 12.5.7, it doesn't have the hardware needed to run anything higher.
If you need a higher iOS version, you will have to purchase a newer phone.
To update to 12.5.7 -
Go to Settings > General > Software Update.
This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -
Update your iPhone or iPad - Apple Support
This link will show you how to update using your computer -
Update iOS on iPhone - Apple Support