After right-clicking on the existing territory, you will see three options as follow:
– Assign Parent Territory
– Copy To
– Move To
You can assign parent territory to any existing territory from the territory map itself. Once you plot the territory using the “Territory” search option, select that specific territory using the selection tool. 
By clicking on the “Assign Parent Territory”, you will see the popup named + Assign Territory. You need to select the parent territory from the drop-down selection. 
By clicking on the Save button, the selected territory will be assigned as a sub-territory of the selected territory from the “Assign Territory” popup. 
By moving the mouse cursor over the area, you will see the parent and sub-territory.