how do i download ios 13.0

You cannot. An iPhone 6 does not support iOS 13.

iOS 12.5.1 released a couple of weeks ago, is the the most current iOS version an iPhone 6 can run, and contains the most current fixes.

Sorry, you'll need a newer iPhone model for that. The 6 is too old and is stuck on iOS 12.5.1 due to lack of RAM.