How to restore a jailbroken iphone 5s?
You need to boot the phone into DFU mode. 


Plug the iPhone cable into the phone and the computer, with iTunes open.
Turn off the device.
Hold the Power button for 3 seconds.
Hold the Home and Power buttons for 10 seconds.
Release the Power button but keep holding the Home button until iTunes recognises the phone.


The screen will turn on, but it will be black. iTunes will recognise the phone in DFU mode and give you the option to restore.