The path to the section is ....
Settings ---> General ---> Storage & iCloud Usage >
On my iPad the top Section is Labelled "Storage" this is the right one
"Open" Manage Storage > in this section
and you should see the IOS 9.3 update listed if it is downloaded on your device
you can then open it and delete it.
Find the 9.3 update and delete. Then try again and it should work...
Hopes this helps someone else.