How do I reset my return such that I can auto populate my information from CRA (including stock gains/losses)? I skipped this option once, and now can't reset to use it.

If you never entered any of your slips, you can return to Auto-Fill my return menu on the left-hand side of your screen and continue and you will be able to Auto-Fill. If you have entered some of your slips, you will need to start a new return from scratch. 

Thank you for choosing TurboTax.