I don't think you can upgrade to 7.5 any more. Check the downloads section of the Avid web site to find out for sure. At least make sure you have all the updates for your version.
The default organ is only a few flute stops. You can change the registration just as you would the real thing. You'll need to spend some time in the manual. Most people just what a big organ sound. To do this select the first note of the organ part and in Technique Text type "Full Organ" .
BTW, you can also have voices sing "oo" or "ah".
You can also change instruments in the mixer. Ever wonder how that organ part would sound on piano or strings, instead?
Sibelius 7.5, W10,i5,16 GB ram,desktop