Discourse, by default, would have a grace period of 3 days since account creation, in which a username can be changed. The fact you are not able to edit your username, means the grace period set for this forum, has expired, or it has been disabled, not allowing a name change at all.
Generally allowing users to change their username, can be confusing, if you have been around for some time.
A username can only be changed by an admin on this forum. We, as leaders can only forward your request, which likely requires an exceptional reason for being granted such a change.
Let us know, preferably in a Private Message, if you want us to forward your request, including an explanation, and we will bring it to the attention of the forum admins.