1. Go to the Branding section, either via SELL > Branding or via the Account Settings > Business > Branding > Watermarks > Manage section.
2. Click on the watermark you want to edit, then click on the wrench icon.
3. Make the change and click on the SAVE button.
IMPORTANT: the change you make here to the watermark WILL NOT APPLY automatically to those images which were watermarked earlier. You will need to re-apply the watermark to those images in the gallery via the Organizer.