how can I update my iphone 6 past iso 12.5.5 update. Cannot update current apps

Your iPhone is updated as far as it will go - the hardware doesn’t meet the requirements for iOS 15. Developers routinely drop support for older versions of the operating system. You might consider buying a newer model iPhone.

You can only download the app versions that are available in the App Store. Developers drop support for older versions of iOS, as well as update code, correct problems and remove vulnerabilities. It is likely that, if you were able to load a previous version, you would open your device up to bugs and/or vulnerabilities. The only thing you can do now is buy a newer iPhone.