To fill a cell, select the number from the bottom row and click on the cell
To change number in cell, select another number and click on the cell
To empty the cell, select 0 from the bottom row and click on the cell
To check whether the filled cells are correct or not, press the 'Check' button
Press 'Uncheck' button to decolor the cells
Press 'Solve' to solve the sudoku
Press 'Solve (Visualize)' to see the solving process.