The single-Daled knot is made of a vertical and a horizontal strap, when looked at from the outside.
The horizontal strap is woven around the vertical strap.
The vertical strap goes straight down - but is S shaped. IOW it goes down, up again and then down again. But it's not twisted or tangled.
To adjust the straps, you move the vertical strap.
For example, to shorten the headband, you would first pull the back of the S down, then pull up the top of the S and then pull down the long strap.
However, being that the horizontal strap is wound around it, it is usually too tight to be simply pulled. So you may have to loosen the grip of the horizontal strap.
Note: If you loosen the horizontal strap too much, the entire knot will disintegrate and you'll have to re-knot it.
Here's a video I found that explains it better.