If you need to make changes or a correction to your 2019 tax return you should first wait until you receive your Notice Of Assessment (NOA) then make the changes in one of the following ways:

• If you NetFiled your tax return, you can do a ReFile. 
• Sign in to your CRA My Account online, and choose "Change My Return.
• Print and mail a paper copy of the T1 Adjustment form to your tax centre.

I have filed my tax return. Can I still make changes?

how do i change or uncheck first time filing box on my 2019 UNFILED return

how do i change or uncheck first time filing box on my 2019 UNFILED return

You will need to go back to "My Personal Information" you will scroll down near the bottom of the page and answer "No" to the question is this your first timing filing. Continue and Save. Then you can continue your return or if you were done- go back to the "Review" section and try again to file your return.