How do I remove ios15.7 from my iPhone 7

you don't

iOS 15.6.1 is no longer signed so it can't be installed on any iphone7's

twin317 Said:

"How do I remove ios15.7 from my iPhone 7"

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Downgrading from iOS15.7:

You only option would be to restore from a backup of the same iOS version (iOS15) that you made prior to this backup that is of an earlier release (pre-iOS15.7). You cannot downgrade to iOS14 and before.