Where to ask session-related questions (web app)

If a session allows questions, you will see a ‘Q&A’ button and ‘Ask something’ field 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 via the ‘Change nickname’ button under the ‘Ask something’ field.