Could not update iPhone 12 Pro Max to iOS 15

Hello ~ Take a look at this excerpt from the link I posted above…have you tried this?

*Remove and download the update again

If you still can't install the latest version of iOS or iPadOS, try downloading the update again:

1. Go to Settings > General > [Device name] Storage.
2. Find the update in the list of apps.
3. Tap the update, then tap Delete Update.
4. Go to Settings > General > Software Update and download the latest update.

If you don’t see the update in the list of apps or if the issue happens again, you need to update your device with your computer using Finder or iTunes.

*If your iPhone, iPad or iPod touch won’t update. 

~Katana-San~

keilla0103 Said:

"OS 15: Hello. My phone will not let me update to the newest update & because of that I am not able to use my phone unless it’s connected to WIFI. Please help."

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To Check for Updates:

Use a Computer and Ethernet:

Start with attempting an update through use of a computer. If you cannot use Wi-Fi, then use Ethernet.  Go Here: Update your iPhone Using a Computer - User Tip. On the computer, you would connect it through Ethernet, going directly into your router. For a Mac (or PC) that does not have Ethernet, this would be the adapter to use: Belkin USB-C to Gigabit Ethernet Adapter - Apple. If you get the same error, the model is likely just not compatible.

Hello again~ Take a look here and when updating you are connected to WiFi?

If your iPhone, iPad or iPod touch won’t update. 

~Katana-San~

Hello ~ What happens when you try to update? Messages/ errors?

~Katana-San~

How much free storage space do you have on your phone? Go to Settings > General > About, and look next to "Available."