You can only edit the text content of fields that have been configured to be editable. You can identify an editable text field in the working area by the icon that appears in the upper right-hand corner of the field when you click it.
You can edit text in the working area by double-clicking the text field that you wish to edit or clicking the icon in the upper right-hand corner of the selected text. After this, simply select the section of the text that you wish to change and enter the new text in the field.
The amount of text that you can enter in a text field depends on the character limit specified for the field. If your text is too long for the field, you will see a warning in the form of the icon in the element menu. This means that the text will break off and will not be fully visible in the printed materials.