If you want to edit the login screen for a particular project, open the file Login.aspx
in the project's Sitefinity
folder. This is the page that loads when accessing the login for your project. You can either edit it, or create it from scratch and replace it. However, make sure that you keep the functionality in the codebehind file, when making the necessary changes like controls IDs and the like.
If you want to change the login page for all projects created on the particular installation of Sitefinity, the procedure is the same, only editing another file: /ProjectManager/EmptyProject/Sitefinity/Login.aspx.
This file is copied for all your new projects, so make sure you backup first.
the Telerik team