Prise out the old brake block with a small flathead screwdriver.
Spray a bit of WD40 into the metal bit.
Don't forget to insert the threaded metal bit back into the metal holder, you're stuffed if you fit the new block not having done this!
Place the metal holder on end with the 'open' end uppermost.
Ease the new block in a few mms, then insert a larger flathead screwdriver into one of the slots on the new brake block. I think these slots are designed for water dispersal but they also make fitting the new block easier: use downward force to drive the new block down into the metal holder.
That's how I fit my ones anyway ... no vice required and if I can do it then anyone can