Within an existing HTML Element, click on the gear icon to edit.
From the Edit HTML Element window, you will find your existing copy in the Basic tab. You can edit and update your copy there. When complete, click the blue Save Changes button.
If an Element Stack has multiple HTML Elements in it, you will need to click on the toggle icon first. This will display a gear icon within each HTML Element that is contained in the Element Stack. You can then click on the gear icon of the HTML Element that you want to edit.