1. Login to Facebook Using Your Current Password
Once logged in, go to the top right of your page and click on the settings icon.
2. Go to Your "Settings" Page
Click on the "Settings" option.
Clicking on "settings" will navigate you to your account settings page. Look for the first "Username" area on top of all the options. Click the "Edit" button on the right.
3. Change Your Username
You can only change your username once, and be sure it's something that is searchable for when your friends try to find you. The URL you choose will also become your Facebook e-mail address. Authorize the changes by putting in your Facebook password and when you're done, click on "Save Changes."