i cant reset my iphone bcoz someone has been change my icloud password.

HI, i want to ask, I want to reset my iphone 6, due the icloud password has been change by someone, this is sabotage cases. Then now, icloud pop up is always appear at the screen to keyin the password, but i dont no what the password,, at the st when i but this iphone, my friend used his email and password tu setup my icloud, i dont know about this at that time, so now we are had a little argue, then he change the icloud password, Please help me, what should i do? iphone 6 8.1.2. Thanks you.