Tap a cell to give it a number (1–4 fixes its network shape, 5 means off the network), or tap a row or column clue to edit it (e.g. 7 ? 12: numbers or ? separated by spaces; a single − means zero runs; empty means unclued). On a keyboard you can type directly; arrows move, and with nothing selected they step through solutions. Ctrl+Z / Ctrl+Shift+Z undo and redo any puzzle edit.