It depends which shade you have.

Levolor and Hunter Douglas (HD) – have the plastic buttons (stop bumpers) that have knots in them.
The only way to move the button "down" is to untie the knots and remake them more towards the end of the lift cords (to make the end knot farther
down the "line", so to speak.

Most HD's "buttons" come apart and then you can see the cords and knots.
Levolor – it's usually a plastic piece with two holes – one for lift cords and one for pull cords.
Either can be easily pulled through the holes using tweezers.

Avoid cutting the lift cords as they need the whole length for the shade to go all the way down.
Otherwise, you'll need to send the shade away to have it restrung.

If you have Graber (Abbey) shades (they have a button and next to it a flat-looking sliding bumper stop),
you can pull on the "bumper slider" down in order to move it.
It's a bit hard to move it, but it does slide : ).

Hope this helps you, Sheena