I've accidentally set my return as "first time filing". When submitting via netfile, it won't let me because it isn't my first time. Is there a way to change that?

Yes, there is a way to change that. Here are the steps:

1. Open your return

2. On the left menu, click on my info

3. Click on personal information

4. Let's get the rest of your personal info page will open

5. Scroll down to Are you filing an income tax return with the CRA for the first time?

6.  Select No, the CRA has received my tax returns before, either from me or from someone else on my behalf.

7. Scroll down and click Continue to save your change.

8. Click on the Left Menu, Scroll to the bottom and click on File.

Thank you for choosing TurboTax!

Step 5 and 6 is not showing on my end after i click on "Personal Info Summary". Help

If you are using TurboTax Online you will go to My Info- then citizenship. Once on that page, you will see the question "Are you filing an income tax return with the CRA for the first time?" Make sure you say "No". 

Thank you for choosing TurboTax.