how can I remove password from the phone if I have Face ID?

You can’t. FaceID supplements the passcode, it does not replace it. And you will still need the passcode under many circumstances. You might need to enter your passcode or Apple ID instead of using Face ID in these situations:

• The device has just been turned on or restarted.
• The device hasn’t been unlocked for more than 48 hours.
• The passcode hasn’t been used to unlock the device in the last six and a half days and Face ID hasn't unlocked the device in the last 4 hours.
• The device has received a remote lock command.
• After five unsuccessful attempts to match a face.
• After initiating power off/Emergency SOS by pressing and holding either volume button and the side button simultaneously for 2 seconds.