To change your Facebook username, follow these steps:
Log in to your Facebook account.
Click on the down arrow icon at the top right corner of the screen.
Select "Settings & Privacy" and then click on "Settings."
On the left side of the screen, click on "Personal Information."
Click on "Username."
Click on the "Edit" button next to your current username.
Enter a new username. Your username must be unique, and you can only change it once every 60 days.
Click on the "Save Changes" button.
Facebook will then check the availability of the new username. If the username is available, your username will be updated immediately. If the username is not available, you will need to choose a different username.
Keep in mind that changing your username will also change the URL of your Facebook profile page. So, if you have shared your profile URL with others, you may need to let them know of the change.