Instructions
Launch UMDGen by clicking on the program's shortcut in the Windows "Start" menu.
Click on the "File" menu, and then select "Open." Locate the CSO file you want to decompress into an ISO file by using the file browser. Click "Open."
Click on the "File" menu again, and then select "Export." Select "ISO image file" in the drop-down menu, and then click on the "Create" button. UMDGen will now decompress the CSO file into an ISO one.