You can edit your text anytime by simply clicking within it. A toolbar will appear either at the top or at the bottom. 

To the left you can see a set of buttons. Use  to add and edit the text;  to delete the section;  to move it up and down the order;  to add subsections that you can then edit in the same way.

Your edits are saved automatically every 5 seconds and whenever you click outside the section.