How to Update I phone 6

Your iPhone 6 can only update as far as iOS 12.5.6

Here's two different ways to ensure the highest iOS that your phone will support is on your phone.

Update your iPhone, iPad, or iPod touch wirelessly. Click here --> Update your iPhone or iPad - Apple Support

Update your iPhone, iPad, or iPod touch with the Finder on a Mac computer --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support