Doors and Windows - How do I replace rotten exterior window trim?

The bottom part on one of the exterior window trims/casing has a soft/spongy spot in one of the the corners. How do I replace this and what should I ask for (name of item) when I go to Lowes or Home depot to buy new item. Also, should I buy pine trim or manufactured trim. Thanks in advance.

I heard about this epoxy stuff made for this type problem. Basically, you gouge out the rotten wood, mix up this filler and slap it on....once it drys you sand it down to match the wood still left, paint it and you're done.
I am having a forgetful moment right now, but I will see if I can find out what the name of it is.

My sill was partially rotted in one area due to a badly cracked and rotted brick molding. I replaced the rotted brick molding and front sill bib but still had to chisel back to good wood on the sill itself which left me about a 12"x2" gap to fill. Cut a piece of two by four to take up most of the gap and filled the rest with "BONDO". Sanded, primed and painted and you could never tell there was a patch there. You can by Bondo at Walmart, Home Depot, or most any auto parts store. Just remember to cut or chisel back to some decent good would before making the patch.

Your advice was appreciated. Thanks again.