wrote
could someone please give a brief step by
step heads-up on replacing the impeller
only on a stern drive lower unit?

http://www.mercstuff.com/waterpumpinst.htm
You'll probably want to look your pump body over for damage from the dry
run, but you probably won't find any. If your gaskets and face plate are
still in good shape, you can reuse them. Might be a good idea to tear it
down before you go shopping, so you get all the parts you need in one shot.
When you put the new impeller into the pump body, make sure you rotate it in
the correct direction, as illustrated. I find that the hardest part of the
whole process is maintaining pressure on the propshaft to keep the drive in
gear while I slide the lower back onto the upper, but that's mostly because
I seldom have a helper handy. And for gosh sakes, don't forget to put the
trim tab screw back in.