less tension on spring when door raised. Perhaps putting the door up, bracing it properly and working on your seal, stand on a ladder if needed. Much easier than messing with spring.
Get 2 rods that fit perfectly in the holes in the tensioner. The rods must fit snugly and not be tapered, like a pointy pry bar or punch. Unwind the spring 90 degrees and put the second bar in the tensioner then relax the tension letting the rod press against the door header. Both cables should now have some slack in them.
Disclaimer: Do this at your own risk.
Also, I would advise against jacking the garage door at any single points in the middle. These doors need even support at both sides. They are not good at supporting their entire weight against point supports in the middle.
If you've ever seen a torsion spring break you will understand why you don't walk under a door while it's in motion. I imagine an extension spring would be about the same.