Go to Notification Settings
From the merchant portal, choose Settings from the navigation menu, and Notifications from the dropdown menu.
Choose the Notification to Restore
Find the section for Customer Emails or Admin Notifications (whichever one you would like to restore) and then click on the name of the notification, which will be shown in blue text. In the example, I am restoring the Welcome Email.
Restore the Template
After clicking the blue text, the Notification Editor will pop up. Press the red Restore Default button at the bottom, and the template will be returned to its original state (any customizations you have added will be lost).
Save Your Changes
Don't forget to press the Save button.