Undo the build in reverse. Only a screw with a visible head comes out, and every screw you remove goes to the tray.
This is the web version of the Android app, with the first 5 boards playable. Tap a screw with a visible head to take it out.
Colored sticks lie stacked over each other, held down by screws. You can only remove a screw whose head is visible, and every screw you remove goes to the tray at the top. Clear all the screws of one stick and that stick drops out, emptying its slots in the tray again.
Poke around at random and the tray fills up until nothing can be removed at all. Finishing one stick at a time is the way this puzzle is meant to be played. Undo is always available, so getting stuck never means starting over.