Don't think a "user" can change his/her username. Possible solutions:
Use the "Send Feedback" link next to the Forum link to contact the people that run Family Echo and ask them if they can change your username for you.
If that doesn't work you can create a new account with a new username/password. Then download a GEDCOM or FamilyScript copy of you tree and upload it to the new account. If you have pictures attached to people on your tree they will not be downloaded with these copies. If you don't have picture backups you can download an HTML copy of the tree that will contain the pictures.
