Can I update my iPhone 6S Plus to 15.8.3?

Yes, if it is an iPhone 6s Plus – and not an iPhone 6 Plus. The two have similar names, but the 6 Plus is an older model that can only update as far as iOS 12.

Update your iPhone or iPad - Apple Support

Updating iOS: common issues and solutions - Apple Community

You cannot update an iPhone 6s Plus past iOS 15.8.3 to iOS 16, 17, or 18, if that might be what you are asking about.

Edit...

muha130 Said:

"Can I update my iPhone 6S Plus to 15.8.3?"

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Updating your iPhone:

Try updating your iPhone through a computer. If you cannot update it, then it is likely out of date. As for getting the options back(if they existed), you'd have to perform a few things, perhaps leading to a reset. So, try the following, reporting if the update works.

Determining if an Update will Install:

How to do so...

A. Verify Compatibility:

Identify your iPhone model. The first thing that comes to mind in incompatibility being the culprit. So, confirm the model of your iPhone. If iPhone model is incompatible with iOS15 then it is because you iPhone does not meet the hardware requirements for this iOS version(s).

B. If Compatible...

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

Did you have a question?