HOW TO INSTALL IOS13 TO IPHONE 6

you can't it's hardware do not support it

iPhone 6 - Wikipedia

• Current: iOS 12.5.5, released September 23, 2021[10]

if your issue is some app requiring a newer version then it's 100% up to the devloper which ios versions they wish to support

How to contact an app developer - Apple Support

iOS 13 is effectively no longer even in distribution. Any device that could run it can run iOS 15 so that is all that would be available to update to.

So any device that cannot run iOS 15 (or 14) also cannot run iOS 13 and thus it is not available in software update. The iOS and iPadOS software update system is designed to only ever offer a single update choice, which will always be the most current version compatible with the hardware software update is running on. There is no user choice with software update - you get the latest based on your device hardware and there are no additional options.