Why can't I download and install iPhone update?

I have an iphone 4s, currently version 5.1.1 (never bothered to update), and I want to update it to ios 6.1.1 (I have a notification on my iPhone settings).

It says that I must have at least 2.8GB of data available, I've checked and I have this exact amount of data free… However, the button on the software update page saying 'download and install' is greyed out and I can't click on it.

Can someone help, please?

Turn off your cell phone and turn on again. I recommend that you erase more thing of his memory.

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