Well, I went to an apple store and they told me that I have to restore the iphone with VF -something- mode, this is made thru itunes. Now, I have an iphone 5s and I am running still Snow Leopard (OSx 10.6.8) on my mac and it work brilliantly, although they do recommend to do so on a fully updated computer.
But that does not excuse Apple, because let's face it, in recent days they have dropped the ball, on equipment, ideas and software, proof of that is that my mac book pro from the mid 2010 still has an optical audio inlet, the software runs more efficiently than the Yosemite and… well who's going to buy a watch worth 1000 quid with a battery that only last for a DAY, serious?
Back to the original matter in question, here's an article that can help you with your problem.
If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support