How do I reset Face ID without the person who changed it

Password

Apple ID and Activation Lock

iCloud uses Apple ID credentials.

If you do not remember the Apple ID Password then follow the steps given in this Apple Article -> If you forgot your Apple ID password - Apple Support

If you do not remember the Apple ID then follow the steps given in this Apple Article -> If you forgot your Apple ID - Apple Support

If you are the owner or the owner has shared all information with you then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Learn how to remove a device from a previous owner's account.

If you are not the owner and do not have the Apple ID and Password then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their Apple ID.

ashlynn168 wrote:

Someone has changed my Face ID to their Face and I can’t change my password to my iPhone now.

You don't need the Face ID to change the password or the Face. What you do need is the current password to the phone? Do you have that? Or did they change that as well?

If you do not have the password to unlock the phone, start here:

If you forgot your iPhone passcode, use your Mac or PC to reset it - Apple Support

Temporarily use your old passcode when you forget your new passcode on iPhone or iPad - Apple Support