Given the limited info in your question, this answer has great limitations as well.
Preload adjustment is simply a mechanism that adjusts pressure on the spring coils. Increasing preload puts pressure on the spring, slightly reduces the distance between the coils, and results in a firmer/stiffer suspension. The spring, under increased tension, requires more initial force to respond. Increasing preload, reduces the amount of sag.
Much of the opposite happens when preload is reduced. The mechanical mechanism that puts pressure on the spring is physically raised away from the spring (everything is still in physical contact), the spring's coils relax a bit, and the suspension becomes more plush: less force is required to move the suspension. Sag increases.
You state a desire to soften the suspension and reducing preload will accomplish this to some degree. Try reducing the preload--probably all the way in your case as a light rider. However, as alluded to in the comments, a lighter (or heavier) than average rider may need a different spring (less stiff in your case being a lighter rider). In other words, you'll replace the current spring with a different one. What I think in your case is that since you couldn't find any info online and the spring suspension fork carries the same branding as the bike, it's likely to be cheaply made, difficult to find parts--especially one offering choices of springs of various weights---and essentially not worth further time and cost to change out.
