Well, first you need Ryders PSSG Editor here:
http://petar.outer-heaven.net/downloads/. Scroll down until you see the PSSG editor, then download the latest version. Its not an installer, just open the archive and click on the .exe.
Secondly, once you have opened the program, In the top left you will see the 'tab' labelled 'File'. Click on 'File' which will then open a dropdown list, just click on 'Open'.
A new window appears in which you have to navigate to the file you wish to edit/open. Just navigate to the 'sky.pssg' file located in the tracks/circuits/(Track Name') folder, and just double left click on the 'sky.pssg' file.
Thenall you have to do is, export the texture on your desktop, edit the file (Be sure to replace the texture file the same as it was originally), import the texture and overwrite the 'default' one, then click save in the dropdown list as mentioned before, locate the file and just replace it, that's basically it.
However, there are a few other textures you have to replace/edit for the sky, not just what is inside the 'sky.pssg' file.
Its a matter of trial and error, but if you want a detailed explanation of how to use the Ryder PSSG Editor, then I suggest you read the documentation file.
