Updating iOS in older version of iPhone

iOS-Updates Said:

"Updating iOS in older version of iPhone: If I'll update the iOS of my iPhone 6S, will it slow my mobile and will it cause rapid battery drainage?"

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Updating and iPhone 6s:

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:

Use my User Tip:  Update your iPhone Using a Computer - User Tip

iOS-Updates Said:

"Updating iOS in older version of iPhone: If I'll update the iOS of my iPhone 6S, will it slow my mobile and will it cause rapid battery drainage?"

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No. But if do you get errors of sorts afterwards, you can always report it as a bug to Apple. Do so here: Feedback - iPhone - Apple