How do I update safari on my iPhone

Safari isn't an app which you update like other apps. Safari updates when you update iOS on your phone. Your signature suggests you are on iOS 11 on your iPhone 6. Your phone is a nearly 8 year old phone, which can be updated to iOS 12.5.5, but no further. Update your phone to iOS 12.5.5 and you'll get whatever Safari version is compatible at that level. If you need a more current version, then it's likely time to consider upgrading to a newer, more modern iPhone. We are currently on iOS 15.0.2 in the newer phones.