It says error. Software update failed. What can I do?

these support pages outline both the available methods by which the update can be performed - and troubleshooting should you experience difficulties:

Update your iPhone, iPad or iPod touch – Apple Support

If you can't update or restore your iPhone, iPad or iPod touch – Apple Support

If your iPhone, iPad or iPod touch won’t update - Apple Support

iOS update and restore errors – Apple Support

ajda273 Said:

"It says error. Software update failed. What can I do?"

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How To Update your Device:

There are two important steps: Backup first then Update.

1. Backup First: Make a backup of your device using iCloud or your computer. That way you can have something to restore you device from, should anything go wrong with the update installation.

2. Then, Update Using these Links:

• On the Device itself: Update your iPhone, iPad, or iPod Touch - Apple Support
• From a Computer: Update your iPhone Using a Computer - User Tip