How do I change the username of a facebook page of mine?

[?]

Do you mean the actual name of the page? Once you have over 100 likes then you can't change it. If you have under that, then you click 'Edit Page' which is at the top right corner, then go to 'Basic Information' on the left side column. And then you will see the page name, where you can edit it.

[Stumpy]

Not possible
Once you set a oage username, you can't change it.

[harman]

not possible....