Not sure whether this is always so, but on my vans the tensioning mechnism has always been in the top right hand corner looking from the inside of the blind. If you remove the screws holding this corner to the "wall" and take off the end cap piece you can introduce a short, flat bladed screwdriver into the slot at the end of the roller. Push down to "unlock" it and give it a couple of turns against the spring. release carefully and test. If too "springy" re-insert screwdiver and let the spring down a notch or two. If OK replace the screws and job done. Don't over tension as blind flying up out of control can do damage.
Hopefully other more experienced posters will confirm or modify this advice but it has always worked for me