Currently, installing a SFS file in a full hard drive installation (as opposed to a "frugal" installation) is slightly more complicated, and you can't undo it. It involves some steps....
1. Click on the .sfs file in a filer window to mount it.
2. Open a terminal in the mounted directory and enter the following
# cp -a --remove-destination ./* /mnt/hda2/ # sync
3. Click on the .sfs file to unmount it
Note1, the --remove-destination option is essential. If you only use '-f' to force overwrite, it will follow (dereference) a symlink, that can cause unexpected overwrites.
Note2, replace /mnt/hda2 with the name of the partition where Puppy is installed.