Where to ask session-related questions (iOS)

If a session allows questions, you will see an 'Ask a question' button once you open the session detail. You might have to wait till your question is approved. 

You can also upvote questions of other attendees and show your interest in that topic.

Where to change the nickname for asking questions?

You can change your nickname used for asking questions anytime on the Edit profile screen. To get there, please go to More → click on your profile → Edit button in the upper right corner.