Default [Q] HTC One Dual Sim 802d

I got an update of android 4.4 kitkat on my mobile. After downloading when i am installing the update suddenly it shows a red triangle on the screen
and after that my continuously started rebooting. it just shows the HTC logo and then reboot again & again & agian. what can i do now?
My phone is unrooted. Pls help ASAP. thanks in advance.
and SORRY for my bad english.