Templates are updated and changed through the Designer tool. Go to the Template Settings and click on the button to Change PDF Background.
This will open a dialog box that will let you choose the new PDF file.
Selecting the new PDF will update the template to use the new PDF design as the background. The green/digital fields will remain in the same location/pages that exist within the template. Thus if inserting or removing pages, it could result in the digital fields being on a different or position. Following the background replace, the fields may need to be repositioned or moved to the corresponding page.
Note: Making changes to templates that are in use have implications that must be thought through. In some instances, instead of updating the template, you may want to create a new version of the template instead.