how do you re-download ios update

David,

Go to your Settings > General > usage section and delete the update. Then connect your phone to iTunes, re-download the update from there and then install it. This works better than OTA updates. Cheers...

Hi davidfromlin'an,

I'm sorry you are having trouble updating your iPhone. The resource below will explain how to reinstall the update. I've highlighted the key information for you. Click on the link for complete details.

If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support
https://support.apple.com/en-us/HT201263

Put your device in recovery mode and set it up again

1. Connect your device to your computer and open iTunes. If you don't have a computer, borrow one from a friend or go to an Apple Retail Store or Apple Authorized Service Provider for help.
2. While your device is connected, force restart it: Press and hold both the Sleep/Wake and Home button for at least 10 seconds, and don't release when you see the Apple logo. Keep holding until you see the recovery mode screen.
   
   
   
3. When you see the option to Restore or Update, choose Update. iTunes will try to reinstall iOS without erasing your data. Wait while iTunes downloads the software for your device.
   If the download takes more than 15 minutes and your device exits recovery mode, you'll need to repeat these steps and choose Restore instead of Update when you get back to this step.
   
4. After the Update or Restore completes, set up your device. If you previously backed up your device with iTunes or iCloud and had to restore your device in step 3, you might be able to restore from your backup during setup.

Get more help

Contact Apple Support if you can’t update or restore your device with recovery mode, or you can't use recovery mode because of broken or stuck buttons.

Take care,