If i install my iPhone 6 to IOS 15 update what happened to my iPhone?

An iPhone 6/6plus can't be updated beyond 12.5.6, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

You cannot download ios 15 on an iPhone 6, that one is stuck on 12.5.6

On the iPhone 6S you can download iOS 15 and I recommend doing so.