How to enable/disable UIBarButton based on the data reloaded in table view?

I am developing File Manager Application where I am listing all Directories in a table view. On selection based on what user has selected I am reloading content of the table view. (For eg User will be showed a list of Directories When he tap on lets say Music Directory I will load all Directories in Music Directory.

Now what i really need to do is i have back button on top But i want it to be enabled when user is not in Root Directory. Where should i place my code to do that ?