Unable to update OS in iPhone 6

Hello. The latest version of iOS your iPhone can run is 12.5.5. If you want to run a newer version of iOS, such as iOS 15, you will need to upgrade to a newer device that supports it.

The latest version of iOS for an iPhone 6 is iOS 12.5.5 which was released recently

You can update to this following the steps at Update your iPhone, iPad or iPod touch – Apple Support (UK)

Sadly your device is unable to upgrade any further than iOS 12.x

iPhone 6 cannot be updated past iOS 12. There will be no updates anymore.