Adjustment of the spring tension of blinds is not too diffiult, so long as you can get to the end of the top roller fairly easily - sometimes they are almost touching a wall.
If you can get to it, remove the screws holding the top (usually right hand) corner to the wall and ease it forward such that you can get the flat blade of a short handle screwdriver into the slot at the end of the roller. If you push down on this i.e. toweards the roller, you should feel some spring tension trying to revolve the screwdrive. Turn to increase this tension and then let the screwdriver move outwards axially with the roller, theis will lock the tensioning device. If this improves things then either leave it alone and refix the blind ( if now going up and down ok) or add more or remove tension appropriately - you don't wnat the blind to snap bak up onto the roller too fast.
Hope this helps, if you search previous postings you will find ( assuming our search facility is more or less working) other and possibly clearer explanations.