Even if you don't have knowledge of HTML or CSS, you can still make some basic changes to the system email copy
Getting There
Make sure you have access to the Duel Dashboard, then navigate to Members > Group profiles > Select the group profile > Email communications
See the following video for a breakdown on tips and tricks to edit pre existing copy
For detailed walkthrough of editing the Digest specifically see the dedicated article.
You can also take advantage of the built-in variables to dynamically input data. You can find a list of applicable variables highlighted in purple under each email's custom template.