It appears that the spring on my up & over Garage Door has loosened, and needs to be re-tensioned, so as I it can be opened.
How is this done?

John Warren, November 2011

this maybe of help to you if a HENDERSON door google henderson garage door cable cone change.
you will need to gain access to the giant spring that runs across top of garage door, may need some help to lift the dead weight of door to get in.
READ the down loaded ,instructions carefully and only WHEN you FULLY understand how to retention cables,attempt/try to retention the cables.
get it wrong you will have either broken fingers, or wrist the long 6ft spring carries a awful LOT OF TENSION .

wirecutterman, November 2011

I know on mine, it cannot be adjusted, the springs hook on, the other end hooks to the cable which connects to the door. Why would you need an adjustment.

SD, November 2011