Here you go... now illustrated.
Fresh from the Hazard Freight:
How she looks without her packaging:
Let's get those screws out and crack her open (after removing the screws, flip it over)
The inner round plastic cover over the coil spring can come off, but if you don't need to do this (like to retrieve the end, if it disappears from view), DON'T. If the oil-shmeared coil comes out, you're going to have a bad day. Note, it's not complicated. The very inner layer of the coil is just bent at an angle to slide between the blue plastic spindle halves.
If you want to replace the line with dyneema, 330 cord, whatever... remove the whole black reel, sliding it off the spindle, and flip it over - you'll see the knot that secures it:
Remove the original knot & cord, replace with your own if you want. Take out the thumb-button, lock & spring mechanism. You don't need it.
Here's 2 tips for putting it all back together:
1. Use more cord than you want to. Like, tie some of the original leash cord to the end of yours after its all wound back on the reel. If it won't fit, that's really ok for this - we're going to remove it in a moment anyway.
2. Use needle nose pliers to guide the inner bend of the coil back on the spindle when you slide the reel back down on the bottom half of the case.
Once the spring is anchored in place, pull the extra length of cord out and lay your intended cord through the outlet like the leash originally was. Clothespin, knot/anchor it outside that port somehow. The object here is to create some tension in the spring instead of leaving it "loosey goosey" just plopped back in. This will help rewind your cord.
I tied a loop in the end of my orange 330 cord to attach to a carabiner. I made sure the knot was big enough it wouldn't pull back inside the leash case, then used some extra leash to wrap around the reel a couple more times. When I'd tensioned the spring and got the knot outside the case, I snapped the halves back together and removed the extra leash.
Replace the halves together, replace the screws. You can see from the pictures above there's plenty of room to cut the leash handle off. We don't need it. I ran an extra bit of cord through the lock-button hole and tied it in a loop I can just hook on my saddle or pack. Then I wrapped mine in camo gaffer tape (which is already starting to come off and will have to replace it, maybe spray-paint it... dunno yet.
Voila, as they say.