Please how can I upgrade my iPhone

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

If your iPhone is an iPhone 8 Plus, running iOS 14 – as your signature line indicates – then you can upgrade to a later version of iOS. I suspect that you won't be offered a choice of upgrading to iOS 15, only one of upgrading to the latest compatible version of iOS (i.e., the latest point release of iOS 16).

iPhone models compatible with iOS 16 - Apple Support

Upgrade or Update?

See: