Allow user to Edit/Change their Username
> allow members to change their username
I don't think there's a way to do it out of the box, but you could certainly code a solution.
> or if there's a way to do it by admins through the dashboard.
Just edit the user.
I have a multi-level member site where I am only capturing first name and email address to start with, (email for login checked in dashboard) and then capturing more information when they pay.
In the paid areas of the website, there are addons like the guestbook that display the username, so ideally I would like to allow users to create a username at this second stage and have the username replace the one originally created when they joined.
The other option is to replace the use of the username in the guestbook and other functions displaying the username, but the above would be ideal.