Hi there, to delete the most recent version of a dataset in your Library list, just click the red X button for it:
if you want to delete *all* of the versions for the dataset-- for example in my screenshot above you'll see I have seven versions of the dataset-- click the All Versions
And you're drilled down into a page that lists each version of that dataset. Now notice the red button to DELETE ALL VERSIONS:
If you are sure that's what you want to do-- to delete every instance of this dataset from your Library-- then click the button and, poof!, they are gone.
hope that helps!