Once you have placed a tile, it can be removed using the Undo button, or the table cells themsleves can be swapped by dragging. Press Restart to try again, or New for completely new tiles.
There are three levels of difficulty, with each increase in difficulty meaning more tiles and a greater variety of values.
Hint: the target value will always be either zero, or either the highest or lowest value possible. Try and use this knowledge to help decide the best way to place the operation tiles.