Can’t download the update

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

jeramie81 Said:

"Can’t download the update: need to download the new update"

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What update?

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 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

jeramie81 Said:

"Can’t download the update: need to download the new update"

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Downloading an Update:

To officially want to "download" the update (not "install" it), you'd have to go into apple.com, and search for the update. Then, perform an install.

If you are asking about iOS 16, it is not yet available.