You can also manage the existing territory by copying or moving to another of any search options. [1]
Copy/Move Territory: Right clicking on selected region/shape that is an existing territory, it would show option to copy or move territory. [2]
Clicking on Copy to/Move to would open a dialog box to select if territory must be copied/moved to an existing territory or create a new one. [3] [4]
In case you are moving/copying the territory by creating a new one, it would ask to enter territory name followed with manager selection for the territory.