The easiest way, if it works with your door, is to put a solid block under the side of the door with the cable off the wheel. Put the door down so it flexes the other way, loosening the cable on that side. climb up to the top and put the cable back on the wheel. If this doesn't work, not enough slack, you will need a rod that fits the holes on the spring end and tighten the spring so you make the cable slack more and put it back on the wheel. You might need 2 people on ladders for this second method. One of these will work. The first, if it works, is easier than the second.