How do you revert bloody nightmare difficulty to nightmare difficulty after you pick up the head

I don’t think you can. All the buttons in the basement of the nexus break, don’t they?

Return to the Nexus and head down in to the basement. Look to the ceiling and shoot the red pentagram. This will open a door leading to the standard Nightmare difficulty button that will disable Bloody Nightmare.

However if you're in the full Bloody Nightmare playthrough (after killing the last boss after picking up the head) then all difficulty buttons are disabled down in the nexus.

Thanks

Only in NG+ mode (when after defeating Chthon on BN difficulty you get ported to the beginning with all upgrades and items removed)