Open the desired document in the tree structure on the left. By default, the "Edit" tab should now be open in the menu bar.

Now the page should be displayed as it looks online. You can easily click on the text you want to edit and make the changes.

If you are no longer sure what is editable on the side, the light bulb on the left side will help you. By clicking on it, the editable content is highlighted in yellow.

If elements or text passages are "locked", you may find them in the objects or in the general website settings.