Go to Inbox...Manage Preferences
Select the Behavior Prefs tab then Reminder/Letter
Click the Header/Footer button at the bottom right
Edit the note
A likely edit that many may want to make is to remove the Dear John casual salutation. You can simply delete that selection.
Here's the note after the Dear John is deleted
Inserting other standard text
If you want to insert some different text, you need to put the cursor on a blank line and then select the desired text from the drop down.