If your comparator works in ascending order, then all you need to do is to invert the result of your comparator.
If this code works for you:
return numfromS1 - numfromS2;
then change it to:
return numfromS2 - numfromS1;
or, maybe to the little more idiomatic:
return (numfromS2 < numfromS1) ? -1: (numfromS2 > numfromS1) ? 1 : 0;
and now your data is going to be in descending order.
You could also, as suggested by others, use either:
return ((Integer) numfromS2).compareTo(numfromS1);
or:
Collections.sort(data, Collections.reverseOrder(aStringComparator));