This simplest way to achieve this is to modify the localization file(s) and change the value for $labels['username']. This change however, has be repeated after every update of Roundcube and should be applied with a patch file. A more complication approach is to create a custom skin which replaces the login.html template. The downside here is, that the login form itself is created by an internal function and would need to be re-built by hand inside that login template. See
https://github.com/roundcube/roundcubemail/wiki/Skins for details about this.