How I can update my iPhone

Normally, you can update an iPhone by selecting Settings > General > Software Update. If your phone is eligible for an update, you will be prompted to do so. Unfortunately, you probably cannot update your phone. the hardware on that iPhone 6 Plus is only capable of updating to 12.5.7.

An iPhone 6 Plus cannot update past iOS 12. If you need a newer iOS version, you will need a new phone.