How to install iOS 13 on iPhone 6

You cannot. an iPhone 6 does not support iOS 13 or later.

iOS 12.5.1 is as far as it can update. 12.5.1 released a couple weeks ago is updated with the most recent fixes.

You cannot install iOS13 on an iPhone 6, or any other phone for that matter. 12.5.1 is the highest for a IP6. If you try to update a newer phone than that, it will go straight to 14.4.