how to change apple id without password

 you will need to contact the previous owner to have them log out of iCloud or go to iCloud.com/Find My Phone and erase the device, then remove it from their account. If you can’t do that, the device is useless. Apple will not unlock it unless you have a copy of the original purchase receipt with the serial number on it.

Activation Lock - Turn off Find My iPhone 

For obvious reasons, yes, you must know the password used for the AppleID to sign out of icloud and find my iphone in order to disable activation lock. If that previous owner will not or cannot help you, you will never be able to use that device yourself.