Change "Note to self" email address
Ok, I finally found the answer. The option was a lot more buried than I thought it would be. 


Open the Google app on your phone.
Press the More (hamburger) menu button, then choose "Settings".
Under "Google Assistant", tap "Settings".
Switch to the "Assistant" tab, then tap "Routines".
Scroll through the list and find "Take a Note" or whatever your command is and tap on it.
Under "My assistant should...", you should see, "email ". Tap on the gear next to it and edit the email address.
Tap then back arrow, then tap the check mark to save the change.