Go to the link https://www.playstation.com/en-us/support/system-updates/ps4/ and make sure you are downloading the righ file, which is the 850 MB one. Next, you will need to format your USB to FAT32 and then create a folder called PS4 and another one called UPDATE, move the 850 MB file into the UPDATE folder and the UPDATE folder into the PS4, and you're good to go.
Insert your USB storage device and turn your PS4 on, and select the last option to install the system software.
Hope I helped.