Hi, I am trying to find out where to edit the off-canvas nav description. I've attached a screenshot to help. Thanks a lot.

Sorry I found it - for those who are after the same thing. 

Go you your menu page > click on screen options at top of screen > click on the descriptions check box. 

Now when you open up each menu item you will see a description field that you can edit to create the nav description. 

Have fun!