How to remove an installed IOS

How to uninstall iOS beta software.

How to uninstall iOS or iPadOS beta software - Apple Support

Then you can install iOS 15.6.1.

Youngfuture Said:

"How to remove an installed iOS: I want to remove my installed IOS 14.7 version from my iPhone,how do I go about it?"

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Downgrading an iOS:

You Can't. What you are looking to perform is not an option for iOS. You cannot downgrade to a lower iOS version.