Why can't I unjailbreak my iphone 5?

I want to update to 6.1.2 but I'm jail broken with evasion and when I connect if go my computer to restore it takes a long time to load and says that it's up to day or something and when I check my software update on the iPhone it never loads and when I tried resetting it it never finishes what can I do? How do I unjailbreak when nothing's working

Take it to a shop

Alright. Well all I could tell you is. Kill your iphone so the only way to fix it is restoring it & to do so try to put it in DFU mode but instead of holding the home button for 20 seconds keep doing it until you see the itunes logo & the usb on your iphone screen after that it should say that your iphone is on recovery mode once it says that just restore it with the new firmware.

Ok first of all… You're a dumbass for jailbreaking a top dollar, best selling, one of the most advanced phone in history. Try formatting the memory and restore it with your computer. I think you can take it to an apple dealer and they can fix it for you.