deleting parts

OS: Windows 10 (10.0), Arch.: x86_64, MuseScore version (64-bit): 3.6.2.548021803, revision: 3224f34

Hi all
Years ago I wrote an arrangement which was, to put it mildly, a bit ambitious for my big band. So just recently I've copied the arrangement into a new file and really simplified it and certainly made it visually less intimidating. However the parts are all screwed up. So I made some new parts which seem fine. But now I've two sets of parts.
My question is, how do I either delete one set of parts or failing that, all the parts?
Thank you.