How do I cap off a HVAC duct?

I am renting a 3 bedroom 2 story home and it gets VERY cold downstairs and VERY hot upstairs. The AC unit runs 24 hrs to try and cool upstairs but never gets it below 80F. Last night I was in the kitchen and felt really cold air blowing on my feet from under the sink. I pulled the board off under the sink and found a huge cement pipe about 12 inches in diameter that is wide open and blowing freezing cold air. This vent is also the closest to the AC unit. I assume if I can block this off I can allow more air to be forced upstairs and finally cool my bedroom. How do I properly plug this off? Right now I just have a piece of wood covering it.