I want to update my iOS to iOS 15.5.1

Ominisan Said:

"I want to update my iOS to iOS 15.5.1"

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Updating an iPhone 6s to iOS 15.5.1

Only the latest-compatible version would install.

Update Using a Computer:

1. Back Up First:

IMPORTANT: If you have an option to update, however, then be certain you backup your iPhone first. That way, you will have something to restore your iPhone from should anything go wrong with the update

then...

2. Proceed with the Update:

Go Here:  Update your iPhone Using a Computer - User Tip