1/ Double-click the cell you want to edit.
The background color changes to white. The possible values appear in it.
2. Edit it to leave the requested number only.
You can also click one of the buttons under the table:
Click "Back" to cancel last entered number.
Click the red "Reset Board" button to start a new puzzle.
Click "Solve Now" to let the computer choose numbers.
This is useful when the trial and error strategy is required.