Log into your account and follow these steps:
1. Select the menu icon on the top right of the page (to the right of your picture), and under the menu that appears choose Settings, then Account Settings:
2. On the Account Settings page, select 'change' next to Username to update your username. Your username can not be in use by another member or contain any spaces. If the username you like is taken, try adding a number or a last initial to make it unique.
3. Enter your password or follow the prompts on the Facebook pop up. Then select 'change username'.