iOS 12.5.6 Can be upgraded?

An iPhone 6 can be updated to 12.5.7, but not higher.

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

Unfortunately it cannot on your iPhone 6 Plus as support ended after the final iOS 12. You can check here what models support certain updates for all iOS: Supported iPhone models - Apple Support

Yes!

The current version of iOS 12 is iOS 12.5.7.