In principle, for the default text to disappear, you simply need to type over it. Just place the cursor on the line and start writing.
In case you don’t want to add text where there is default text suggested (most common the pictorial layout), just place the cursor on the line and hit the Backspace key on your keyboard.
On the Cover or Image layout, you can remove the text block altogether by clicking the “X” button that appears on the right as you hover the text block. You can add the text block again after that, via the orange dot on the left side.