This article explains how to change your password on Facebook. It covers how it works if you're already logged in, and how to reset your password if you can't log in.
How to Change Your Password on Facebook
Follow these steps to change your Facebook password from a desktop web browser. These instructions also work for the mobile app, but the screenshots are from the Facebook website.
Select your profile picture in the top right corner, then select Settings & privacy > Settings.
Select Accounts Center.