Can not update system to 15.3

15.3 is no longer available. If you really have a 6S plus, then you should be able to update to 15.7.1.

Update your iPhone, iPad, or iPod touch

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.6

• If you have an iPhone 6S through 7, you can update to 15.7.1

• If you have an iPhone 8 or higher, you can update to 16.1.1

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support

If you are having issues updating, then what happens when you try?

Can you post a screenshot of the whole screen, it could reveal why you aren't able to update.

Make sure you:

• Are using a WiFi connection (not a hotspot).

• You have an active cell signal (You need both WiFi and cellular service to perform the upgrade).

• Are not using a VPN connection.

• Have your battery charged at least 50%. Better yet, keep it plugged in during the update.

• Do not have power save mode turned on.

• Do not have a beta version installed.

• You have at least 10 gigs of storage available.

Click on this link on what to do if you can't update your phone -

If your iPhone or iPad won't update – Apple Support (UK)

"Can not update system to 15.3: Can not update system to 15.3"

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For reference...

Updating an iPhone:

A. What to Update:

Update Installed Apps along with the iOS.

B. Determining if an iOS Update will Install:

How to do so...

I. 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).

II. 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

浏芳 wrote:

Can not update system to 15.3

That’s true, but you should be able to update it to 15.7.1. Go to Settings/General/Update and follow the instructions.