The command you're looking for is mv (short for move), this should do it:
sudo mv /usr/pluto/bin/ripDiskWrapper.sh /usr/pluto/bin/ripDiskWrapper.sh.old
Then copy the downloaded script over, supposing you are in the same directory as the file, and it already has the right name:
sudo cp ripDiskWrapper.sh /usr/pluto/bin/
Or alternatively, for instance if it was put elsewhere or under a different name:
sudo cp [<path>/]<file-name> /usr/pluto/bin/ripDiskWrapper.sh
Setting the right permissions:
sudo chmod 755 /usr/pluto/bin/ripDiskWrapper.sh
And you should be done.
The script seems to be working fine for me, altho I haven't checked the actual tags yet. If there's anything in particular you'd like me to try or look at to verify let me know. I am using FLAC btw.