I need to update software on my phone

Use a computer or the Software Update function in Settings. If you encounter a specific issue other than the one listed below, describe it in this thread.

Apple no longer allows 15 or 16 to be installed onto units capable of 17.

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What happens when you go to Settings > General > Software Update?

If an update is available, you may need to free up some storage space on your device to download and install the update.