Q: Why can't I update to iOS 8?

Hey,

I've got a 5c, and I've download iOS 8 to my iTunes (on Mac), ready to install to my phone.

However, every time I go to update it gives me a message like: "This device isn't eligible for the requested build."

I've tried updating everything and switching everything off and on!

What do I do? I'm stuck and it's annoying!!

Thanks!