Hey Richard, just a heads up, I'm not UFile staff, but I've been using their software for a decade.
When you want to amend your tax return, you have to wait until you've received your Notice of Assessment for the year. One of the reasons for this is because the CRA may have processed a change in a return after you've submitted it.
Generally, you can request a change to your tax return by amending specific line(s) of your return. For more information, visit: https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/change-your-return.html