How do I factory reset an iPhone without knowing the Apple ID password?

You're up against this situation:

"Activation Lock for iPhone, iPad, and iPod touch" is a security feature with an aspect of possibly deterring re-selling of stolen devices. To learn more click here --> http://support.apple.com/HT201365

An activation lock can only be removed using information for the account that applied the lock. If you know this information follow the onscreen steps to enter the Apple ID and password used to set up the device, or enter the device passcode.

Click on this link to learn 'How to remove Activation Lock' --> https://support.apple.com/HT201441

If you do not know the required information and you are not the original owner, get the owner of the account to remove the lock or return the device for a refund or recycle it. The device is otherwise useless to you. If the owner is unknown and has not provided contact information as a message showing in the login screen, there is no way to contact the owner unless the person who gave you the device knows them.

If somebody needs help dealing with lost information:

• "If you forgot your Apple ID" [this is likely an email address you are using or used in the past] click here --> https://support.apple.com/HT201354

• "If you forgot your Apple ID password" click here --> https://support.apple.com/HT201487

Otherwise:

- Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Do this online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not state what currently constitutes proof. Prior to this new online service Apple typically required the original receipt issued by an Apple Authorized dealer and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership).

Alternatively Apple Stores used to provide this service but I do not know if they still do.

You can restore the iPhone to factory condition without knowing the Apple ID password using a computer and iTunes, however the Apple ID password will be required to then set up the iPhone again as you'll face Activation Lock. link-> Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Without the Apple ID password there is nothing you can do.

If the Apple ID is your own, then see here for how to recover the password: link-> If you forgot your Apple ID password - Apple Support

If this is a used iPhone, then only the the person that set the password can recover it and remove the Apple ID from the iPhone.