Download the free program UnMakeSIS.
Open the downloaded folder and select Extract All Files. An extracted version of the folder will appear in the same location.
Open the extracted folder and double-click on UnMakeSIS.exe.
Press Open... at the bottom of the UnMakeSIS window and select the SIS file on your computer. All the files that the SIS contains will be listed in the Files box.
Extract the files from the SIS file by clicking Extract Files, next to Open.