4s won't accept ios7 update

my 4s iphone is still on ios5 - the update to ios6 would never install, and i had eventually given up!

now i'm trying to finally get ios7 updated - downloaded the whole bit, go to install annnnnnd it won't. says it cannot be installed on my iphone. what am i supposed to do?

most annoyingly, it has backed up my phone - and now since my phone wouldn't accept the update, it's being restored to original settings. NOT WHAT I NEEDED!

help appreciated, i'm a relatively savvy person but this is making me feel like a tech dinosaur.