How can I update my iPhone 6 iOS?

Go to Settings > General > Software Update. If you see a message that your software is up to date, then you will need to upgrade your iPhone in order to use a newer iOS version. The iPhone 6 can only be updated to iOS 12.5.7

An iPhone 6 can't be updated beyond 12.5.7, it doesn't have the hardware required to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

The iPhone 6 cannot be updated to iOS 13, 14, 15, 16 or 17.

Time for a new phone.

What is your iPhone model ?