Nav Mesh Agent

Hi, i am new in Unity and i am trying that project tutorial https://www.youtube.com/watch?t=1409&v=l86gpYbQFzY . I trying to extend code (for better understanding). So i did "killable" Environment (one of them), lets him dissapear ( same like in enemy ) but i dont know how can i update Nav Mesh Agent field, whenre enemy can step. Know its not any function for that and i can use any other patfinders like http://arongranberg.com/astar/, but i wanna to continue in that tutorial, but with my "upgrades" . Thanks for any tips, sorry for my english.

Lukáš Vašek, Czech republic