I understand that you've downloaded iOS 11, but you've been unable to install it. I'm happy to provide some help for this issue.
Whether you are installing this update over-the-air or using iTunes, the following articles can help:
Get help with over-the-air iOS updates - Apple Support
If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support
Thanks for using the Apple Support Communities. Cheers!