Can I install iOS 15 update on to iPhone 6?

Sorry, an iPhone 6 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

No matter how much you ask, the answer is not going to change. An iPhone 6 does not have the hardware to support iOS 15 or 16