Here is the problem: i modified a form of Skyrim.esm in my.esp, so now i've a * near the Form. Now, if i change my mind and i want to cancel my modifications and go back to the unmodified Form, how i do it?

Open the "Data" Window, select your esp and click on the "Details" Button. Select your changed Form and hit the Delete Button on your Keyboard. Answer the question with yes. Save your PI and reload it.

Thank you! It's what i was searching for