You can't change your username but if you come up with a more creative or better name that you would like to use in the community, you can change your "display" name, the one that all the other members will see displayed.
Here's how:
1. Go to "My Controls" at the top of a community page in the light blue navigation bar.
2. Scroll down on the resulting page and look in the left hand column for "Personal Profile".
3. Select "Change Display Name" and follow the instructions.
Post here or in the Support Forum if you have questions.
