In order to type text in a cell you can hightlight the cell with the cursor and then type the text. The input is finished with pressing enter, tab, the arrow keys, or by clicking on a different cell. When the cell is activated there will be a frame around the current cell.
In order to edit text you can either double click on the text in the cell or click the text on top in the text field.
If you would like to remove all text in a cell you can highlight it and type the new text. The old text with automatically disappear.
If you set a property for a cell e.g. bold style or a background color the setting is valid for the entire cell.