Could not update to iOS 11 because ‘an error occurred installing’
This happened when there is not enough space. 

Before trying to reinstall you need to delete the old update. 

To do this go to Settings -> General -> Storage &amp; iCloud Storage -> Manage Storage. 

Then look for the update and delete it:



Then try to install the update using iTunes:

Open iTunes and select your device 
from the left panel

Click on Summary, then click Check for 
Update

Click Download and Update

When asked, enter your passcode