Downloaded ios7, but how do I install it?

I downloaded ios7 for my iphone but now when I click "update" it starts downloading ios7 again rather than just installing the one thats already downloaded!

What am I doing wrong or how do I get the fully-downloaded one to install?

Waiting to downloaded it again would take hours.

Added (1). By downloaded I mean I clicked the "download only"option rather than download and install

It should have upgraded itself. But maybe you should reset your phone. Nothing will be deleted. Or back it up to icloud then reset. It should be fine after that