On Coursera you don't have a username but when you join the Coursera community you need to choose one to represent you.
You can't change your username by yourself so please choose carefully!
Generally it's a bad idea to change your username because it will confuse people who have been having conversations with you.
If it is really important that your username is changed, please email firstname.lastname@example.org
Note that we can't change anything about your main Coursera profile (and you don't have a username on Coursera). You can edit your Coursera profile information yourself here.
You can edit the other elements of your community profile here