Default I can't install kit Kat update. Modified software.

I have a HTC Desire 601.Put s-on mode, the original recovery installed, close the bootloader but I'm still putting a modified software that when I try to install kit Kat. I need help to update a kit Kat. My phone is from the Spanish operator Yoigo telephony but this released by imei. Thank you and sorry for my bad english!

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