I am considering becoming a Vampire Lord after the Dawnguard quest was finished but when I transformed I tried to revert back but I couldn't what do I have to do to revert back to Human form, and I have to ask you if you are Human if you have a Dawnguard add-on will the citizens of a Hold attack you, and can they sense you as a Vampire because I was reading that the Vampire Lord has stages like the normal Vampirism disease.
If you have Dawnguard installed you won't get attacked at stage four anymore at all, not even as a regular Vampire. To deactivate Vampire Lord mode use the "revert form" power (it should be in the menu with bats, vampire sight etc).
I think that it might be helpful to explain that to access the Vampire Lord abilities menu, you must press the UP or DOWN button on the D-pad. On PC I would assume it would be the same key that you press to select your 'favorites'. The B/menu button merely takes you to the Skills tree. I must admit that this took me a while to figure out. I believe you are asking how to revert form not how to cure.
On the PC the option is under Favorites (default key to access that menu is Q). In Vampire Lord form the Favorites menu has a list of powers inherent to that form (Bats, Revert Form, Raise Dead, and Vampire's Sight) as well whatever powers from Vampire Lord perks you earn. It took me a while to figure that out as well. Harkon fails as a tutor.