icloud trouble

I got my grandmas old iphone. I want to reset the phone but grandma couldnt Remember her password to her icloud account that is logged on this iphone. she tried wrong password too many times and the account got locked. The phone said That the icloud account is locked and to open it up, you have to change password. I've done that a million times but the phone want me to change password again and again! How do i unlock IT?