Only superusers can customize notifications.
On the Settings page, under Setup, go to Notifications. Each assessment series can have a custom set of emails. Choose the assessment type you want to notify about, which event, and the role of the person you are notifying. Then start typing in the subject and body fields to put your own custom messages.
Each different notification email can be individually enabled or disabled. By default, they are all enabled. To disable some of them, select the appropriate values for series, event type and role, uncheck the “Enable Notification” checkbox and click Save.
For your reference, a list of available substitution variables is listed at the bottom of the page. To use a substitution variable, type the entire name of the variable as it appears in the reference chart, in all capital letters, starting with a $ dollar sign.
See also: Working With Notification Variables Notifications: What Causes Them to Trigger?