How to update 12.4.2 version

There are no updates for 12.4.2.

A iPhone 6 can't install iOS 13.

You can't. 12.4.2 was only released for devices incompatible of supporting iOS 13.

You would need to buy a new iPhone if you want iOS 13. iPhone 6 cannot run it.

Download it from your iPhone.

No one here can send it to you.