There's a little inexpensive tool for that; its a 5 inch long bent
prybar to reach under the lip of the interlocking siding and pull them
apart; but i've always just done it by hand. Find an end and push
down and out, once you've got a lip disengaged you can continue down
the seam with your hand and slide it along to detach the whole length.
Before reshaping and reappying the siding around your trim, install the
necessary J trim and/or undersill flashing; see your other openings
(windows, doors) for a sample of what those pieces look like.