In a pinch you can use a regular pair of pliers, if you grasp the chain into an n-shape with only the quick link at the top. This'll stop the chain tension from pulling the link back into shape. Squeeze the quick link diagonally to shorten it and you can pick the two halves apart.
Internal circlip pliers would possibly be a neater solution, since you can squeeze the bushings together instead, but not everyone has them.
Replacing is easy: put one half on each end of the chain, slot them together, and pull hard. Use your thumbs to flex the chain sideways each side a bit and the quick link will click as each pin end seats itself in its slot.