You can choose between two different templates for email reminders, default and plain text. You cannot upload your own template or change the HTML of the existing templates.
Steps
From the main menu, go to Workflows, then
Click on the Workflow you want to update.
Select the reminder you want to edit, then click the Template tab,
Select which template you want to use (default or plain), then
Click Save.
Future reminders will use your selected template.