If you no longer want a cell to have a specific function, you can restore it to be a default cell. You can add a symbol, label or any action to a default cell.
Edit the cell (F5).
Tip: You can increase or decrease the size of a cell. To do this, (in Edit mode) click on the cell and then click on one of the black squares that appear. Press and hold the mouse button and drag the cell until it has the desired size.