Here is the Manual:
http://files.filefront.com/FPS+Creator+Manualpdf/;11233020;/fileinfo.html
To change the skybox:
1. Open Fps Creator
2. Go to File > Build Settings
3. Click on the Level Settings Tab at the Top
4. Click the Edit button next the the Global Script Section
5. Look for Sky in there, change it to your desired Skybox.
If you mean like the Title Screen,
1. Open Fps Creator
2. Go to File > Build Settings
3. Click on the Level Settings Tab at the Top
4. Click the Edit button next the the Title Screen Section
5. Look for Background in there, change it to your desired Background Image.