121 provides several easy ways to add new top and child level terms.
Buttons on the tool bar at the top of the screen
Insert menu on main menu bar
Ins and Shift/Ins keyboard shortcuts
Right mouse click on a selected term in the terms list
This short video demonstrates adding a new top term and a new child/narrower term to an existing structure.To view full screen use the expander button at the bottom left for Windows and Android or top right for Apple products.