Every account has a distinct username/handle, which is used to access your profile. A separate displayname can be set and changed under profile.
Currently, the username/handle can not be changed, if its in popular demand, this feature will be implemented in the very near future.
If you really want to change your username, no problem, just let us know in support, or via dms to the admins.
To change your password or email, access settings by clicking the header profile icon and select "Account".
You can also log out from everywhere by clicking the "Logout from everywhere" button found under account settings.