NOTE: I have NOT tried this myself. Not sure if I am coordinated enough. Just want to keep the instructions in case I ever want to try again. 
From Dave:
This is not as hard or time comsuming as everyone makes it sound.
All you have to do is remove the screws and separtate the two sides just a little, use a flashlight to spot the spring side then take the center device and push it towards the spring and remove the other half of the cover. Now take the wheel and spin it(without removing it)to cause the spring to retension just a little then with your thumb holding the wheel – to keep the tension feed the cord back through the hole and tie a knot to keep it there. Now carefully
replace the cover while keeping the wheel in place and replace screws. So abra-kadabra it’s fixed reattach the main lead and it’s as good as new just a little shorter.
From Jeane
I have fixed several. Saw there were good instructions for getting into the ‘box’ and re-tensioning it but remember, you do have to reattach the leash end. A knot has never been satisfactory. I finally learned to take the old string out of the little plastic thing near the ‘leash end’ -observing the threading and then put the new end from the now fixed lead back in. This also takes some effort and coordination.
Cool.. I have always wanted to take one of these apart to see what it looks like inside.
so…cynthia, have you taken yours apart yet?
I’ve got an old one in the basement I’m thinking of tackling tomorrow just for the fun of it. 