By dragging a value, a candidate or a color

By selecting a value, a candidate or a color and then lightly pressing the cell to modify

In mode delete, you can remove candidates if you think they are impossible.

Redo it if you think a value is now possible.

You can also cancelling a value by dragging the cell 'delete'.

This will add a new help-step and will change the candidates in is row, bloc or column.