As John mentioned, you can modify the label and textbox of the username through the html file that is placed in widget.
I believe there is a loginform.html file under template folder.
Did you try to modify the login widget?
Search for the Login widget and change the captions in the properties accordingly.
If you mean this page:
Then copy the login.html from you deployment directory (deployment/web) to you theme directory (if you using a theme which is in a zip, extract the contents and set your theme in the modeler to default.). You can adjust you login.html now. On next deployment it will use you changed login.html: