There are a few easy ways to do it.
First, if I simply miss a tax slip which only arrives after I file my tax return, I can log in to CRA My Account online and go to the tax return tab and click the change tax return link. There are 2 ways to do it. One is to change certain line in the tax return which I found extremely difficult. How would I know? The other one is easy if the missing tax slip is also in CRA My Account. Sometimes, it is not. I know certain people have some tax slips that are not in CRA My Account. Perhaps the employer or financial institution didn’t send to CRA. I can just load the tax slip that was not reported. Then, it is done.
Second, I use Wealthsimple tax. It is very easy to change my tax return after I already file it. I just need to sign in and click Refile and update whatever info I need to update. I can load additional tax slips, claim more expenses or correct certain figures. Very flexible.
Third, I can download a T1 Adjustment form online. It is fillable PDF file. I can write down what the changes I need to make and then mail it to CRA. The mailing address is on the second page of the form.
No need to be anxious. Piece of cake. I know some are still working on it. Deadline is end of this month. Hope you get your tax done soon.