Can I update my iPhone 6 Plus?

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

An iPhone 6plus cannot be upgraded past iOS 15.7.9.

Update your iPhone or iPad - Apple Support

A 6s Plus can run 15.7.9, but a 6 plus can not.