Hi Ohm,
Depending on exactly what and how you want to export, your queries could be a bit different.
But basically, just click Manage Queries and create a new query based on the Portfolio Item Feature. Then, add a Filter for the Parent = Your Epic. Something like this:
Then, click OK. Then select the query you want to use, then click Run Selected Query:
Same for User Stories, just a bit different. The difference being of course that you are exporting Type: User Stories, and your filter is going to be on Feature = Your Feature. Like this:
Let me know if that helps.