Not updated our software which reason

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Go to Settings > General > About

What does it say for iPhone Model and Software Version?

You need an iPhone 6s/6s Plus or newer in order to run iOS 14/15..

Supported iPhone models - Apple Support

If your iPhone is an iPhone 6, then you cannot upgrade beyond iOS 12

have you read thru this support article for updating?

Update your iPhone, iPad, or iPod touch - Apple Support

need more storage for update?

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

issues?

Updating iOS: common issues and solutions - Apple Community

video..

stanly21 Said:

"Not updated our software which reason: Software update is not available which reason"

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Determining if an Update will Install:

How to do so...

A. Verify Compatibility:

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