Default [Q] How Do I Return AT&T HTC One (M8) to Stock?

I installed the CyanogenMod development preview ROM a while back, and, as I've learned, that was a BIG mistake. How would I return my AT&T HTC One M8 to stock settings?

Keep in mind that the only reason I was actually able to put CyanogenMod on my phone was many YouTube videos. Please try to explain things to me like I am a baby. Thank you!