To navigate to the export option for a questionnaire template's responses, you'll first head to your list of Questionnaire Responses, which you'll find in the sidebar here:

From there, you'll choose the questionnaire template you wish to export responses for from the dropdown here.

At the top of the page, you'll now see the option to Export Responses.

It'll take a moment to prepare, during that time you'll see the status as processing:

You'll then see a download button when your CSV is ready: