I Just Changed Facebook Page’s Username. Now URL of Opensource Book page on Facebook  is not www.facebook.com/OpenLAMPbooK but now it is www.facebook.com/opensourcebook  You may also do so just follow the following steps:

1. Login to your facebook account.
2. On the left sidebar, just above the chat faces click the Page of which you want to change the URL.
3. On the top you will see  three buttons: Edit Page | Build Audience | Help
4. Click Edit Page 
5. Form the drop-down menu select Update Info.
6. On The Third line You will see Username 
7. On the right side of Username click Change username link
8. You will seeing your old Username side by you Page name.
9. Change it, type the new one and click Check Availability.
10. That’s all. Save the new Username and start using it.