Why can't I update my iPhone?

Jnyut Said:

"Why can't I update my iPhone?"

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Why the Issue?:

It may just not be compatible.

For reference...

Updating your iPhone:

Try updating your iPhone first. Do so, 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 If you cannot update it, then it is likely out of date.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...

Updating an iPhone:

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

no idea, have you read

Update iOS on iPhone - Apple Support