Tap on a cell to select it. Another tap on the same cell will change the selection type. There are two selection types: "Edit the End" and "Edit All". With "Edit the End" selection you can modify the value by adding digits to it or removing digits using "back" button. With "Edit All" selection you can re-type the whole value, or delete the entire cell using "del" button. When you delete the cell, the operation preceding the cell is getting deleted as well.
All your actions are getting saved in undo list. The undo list is getting saved with the calculation.
